快速排序代码（2021 年暑假写）

参考代码 1：

public class Solution {

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 不要忘记了递归终止条件
        if (left >= right) {
            return;
        }

        // nums[left + 1..j] < pivot
        // nums[j + 1..right] >= pivot

        int pivot = nums[left];
        int j = left;
        for (int i = left + 1; i <= right; i++) {
            if (nums[i] < pivot) {
                j++;
                swap(nums, j, i);
            }
        }
        swap(nums, left, j);
        quickSort(nums, left, j - 1);
        quickSort(nums, j + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }
}

参考代码 2：

import java.util.Random;

public class Solution {

    private static final Random random = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 递归终止条件
        if (left >= right) {
            return;
        }

        // nums[left + 1..j] < pivot
        // nums[j + 1..right] >= pivot

        // [0..right - left + 1)
        // [left..right + 1) = [left..right]
        int randomIndex = left + random.nextInt(right - left + 1);
        swap(nums, left, randomIndex);
        int pivot = nums[left];
        int j = left;
        for (int i = left + 1; i <= right; i++) {
            if (nums[i] < pivot) {
                j++;
                swap(nums, j, i);
            }
        }
        swap(nums, left, j);
        quickSort(nums, left, j - 1);
        quickSort(nums, j + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }
}

循环不变量修改以后（代码特别别扭）

import java.util.Random;

public class Solution {

    private static final Random random = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 递归终止条件
        if (left >= right) {
            return;
        }

        // nums[left + 1..j) <= pivot
        // nums[j..right] > pivot

        // [0..right - left + 1)
        // [left..right + 1) = [left..right]
        int randomIndex = left + random.nextInt(right - left + 1);
        swap(nums, left, randomIndex);
        int pivot = nums[left];
        int j = left + 1;
        for (int i = left + 1; i <= right; i++) {
            if (nums[i] <= pivot) {
                swap(nums, j, i);
                j++;
            }
        }
        swap(nums, left, j - 1);
        quickSort(nums, left, j - 2);
        quickSort(nums, j, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }
}

import java.util.Random;

public class Solution {

    private static final Random random = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 递归终止条件
        if (left >= right) {
            return;
        }
        int randomIndex = left + random.nextInt(right - left + 1);
        swap(nums, left, randomIndex);
        int pivot = nums[left];


        // 把 pivot 等概率分到两边
        // nums[left + 1..le]<= pivot
        // nums[ge..right] >= pivot

        // 为了看到下一个元素
        int le = left + 1;
        int ge = right;
        while (le <= ge) {
            while (le <= ge && nums[le] < pivot) {
                le++;
            }
            while (le <= ge && nums[ge] > pivot) {
                ge--;
            }
            if (le >= ge) {
                break;
            }
            swap(nums, le, ge);
            le++;
            ge--;
        }
        swap(nums, left, ge);
        quickSort(nums, left, ge - 1);
        quickSort(nums, ge + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }
}

import java.util.Random;

public class Solution {

    private static final Random random = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 递归终止条件
        if (left >= right) {
            return;
        }
        int randomIndex = left + random.nextInt(right - left + 1);
        swap(nums, left, randomIndex);
        int pivot = nums[left];


        // 把 pivot 等概率分到两边
        // nums[left + 1..lt] < pivot
        // nums[lt + 1..i) == pivot
        // nums[gt..right] > pivot

        // 为了看到下一个元素
        int lt = left;
        int gt = right + 1;
        int i = left + 1;
        while (i < gt) {
            if (nums[i] < pivot) {
                lt++;
                swap(nums, i, lt);
                i++;
            } else if (nums[i] == pivot) {
                i++;
            } else {
                gt--;
                swap(nums, i, gt);
            }
        }
        swap(nums, left, lt);
        quickSort(nums, left, lt - 1);
        quickSort(nums, gt, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }
}

import java.util.Random;

public class Solution {

    private static final Random random = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 递归终止条件
        if (left >= right) {
            return;
        }
        int randomIndex = left + random.nextInt(right - left + 1);
        swap(nums, left, randomIndex);
        int pivot = nums[left];


        // 把 pivot 等概率分到两边
        // nums[left + 1..lt) < pivot
        // nums[lt..i) == pivot
        // nums(gt..right] > pivot

        // 为了看到下一个元素
        int lt = left + 1;
        int gt = right;
        int i = left + 1;
        while (i <= gt) {
            if (nums[i] < pivot) {
                swap(nums, i, lt);
                lt++;
                i++;
            } else if (nums[i] == pivot) {
                i++;
            } else {
                swap(nums, i, gt);
                gt--;
            }
        }
        swap(nums, left, lt - 1);
        quickSort(nums, left, lt - 2);
        quickSort(nums, gt + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }
}

还未优化：

public class Solution7 {

    // 快速排序

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 优化 1 ：改成插入排序
        if (right - left + 1 <= 16) {
            insertionSort(nums, left, right);
            return;
        }

        int pivot = nums[left];
        // nums[left + 1..j] < pivot
        // nums[j + 1..right] >= pivot
        int j = left;
        for (int i = left + 1; i <= right; i++) {
            if (nums[i] < pivot) {
                j++;
                swap(nums, i, j);
            }
        }
        swap(nums, left, j);
        quickSort(nums, left, j - 1);
        quickSort(nums, j + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }

    private void insertionSort(int[] nums, int left, int right) {
        for (int i = left + 1; i <= right; i++) {
            int temp = nums[i];
            int j;
            for (j = i; j > left && nums[j - 1] > temp; j--) {
                nums[j] = nums[j - 1];
            }
            nums[j] = temp;
        }
    }
}

优化 1：加上随机化

import java.util.Random;

public class Solution {

    // 快速排序

    private static final Random RANDOM = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 优化 1 ：改成插入排序
        if (right - left + 1 <= 16) {
            insertionSort(nums, left, right);
            return;
        }

        // [0..right - left]
        // [left..right]

        int randomIndex = left + RANDOM.nextInt(right - left + 1);
        swap(nums, left, randomIndex);

        int pivot = nums[left];
        // nums[left + 1..j] < pivot
        // nums[j + 1..right] >= pivot
        int j = left;
        for (int i = left + 1; i <= right; i++) {
            if (nums[i] < pivot) {
                j++;
                swap(nums, i, j);
            }
        }
        swap(nums, left, j);
        quickSort(nums, left, j - 1);
        quickSort(nums, j + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }

    private void insertionSort(int[] nums, int left, int right) {
        for (int i = left + 1; i <= right; i++) {
            int temp = nums[i];
            int j;
            for (j = i; j > left && nums[j - 1] > temp; j--) {
                nums[j] = nums[j - 1];
            }
            nums[j] = temp;
        }
    }
}

双路快排：

import java.util.Random;

public class Solution {

    // 快速排序

    private static final Random RANDOM = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 优化 1 ：改成插入排序
        if (right - left + 1 <= 16) {
            insertionSort(nums, left, right);
            return;
        }

        // [0..right - left]
        // [left..right]

        int randomIndex = left + RANDOM.nextInt(right - left + 1);
        swap(nums, left, randomIndex);

        int pivot = nums[left];
        // nums[left + 1..i] <= pivot
        // nums[j..right] >= pivot
        int i = left + 1;
        int j = right;

        while (i <= j) {
            while (i <= j && nums[i] < pivot) {
                i++;
            }
            while (i <= j && nums[j] > pivot) {
                j--;
            }
            
            // 这里很关键
            if (i >= j) {
                break;
            }
            swap(nums, i, j);
            i++;
            j--;
        }
        swap(nums, left, j);
        quickSort(nums, left, j - 1);
        quickSort(nums, j + 1, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }

    private void insertionSort(int[] nums, int left, int right) {
        for (int i = left + 1; i <= right; i++) {
            int temp = nums[i];
            int j;
            for (j = i; j > left && nums[j - 1] > temp; j--) {
                nums[j] = nums[j - 1];
            }
            nums[j] = temp;
        }
    }
}

三路快排：

import java.util.Random;

public class Solution {

    // 快速排序

    private static final Random RANDOM = new Random();

    public int[] sortArray(int[] nums) {
        int len = nums.length;
        quickSort(nums, 0, len - 1);
        return nums;
    }

    private void quickSort(int[] nums, int left, int right) {
        // 优化 1 ：改成插入排序
        if (right - left + 1 <= 16) {
            insertionSort(nums, left, right);
            return;
        }

        // [0..right - left]
        // [left..right]

        int randomIndex = left + RANDOM.nextInt(right - left + 1);
        swap(nums, left, randomIndex);

        int pivot = nums[left];
        // nums[left + 1..lt] < pivot
        // nums[lt + 1..i) == pivot
        // nums[gt..right] >= pivot
        int lt = left;
        int gt = right + 1;
        int i = left + 1;
        while (i < gt) {
            if (nums[i] < pivot) {
                lt++;
                swap(nums, lt, i);
                i++;
            } else if (nums[i] == pivot) {
                i++;
            } else {
                gt--;
                swap(nums, i, gt);
            }
        }
        swap(nums, left, lt);
        quickSort(nums, left, lt - 1);
        quickSort(nums, gt, right);
    }

    private void swap(int[] nums, int index1, int index2) {
        int temp = nums[index1];
        nums[index1] = nums[index2];
        nums[index2] = temp;
    }

    private void insertionSort(int[] nums, int left, int right) {
        for (int i = left + 1; i <= right; i++) {
            int temp = nums[i];
            int j;
            for (j = i; j > left && nums[j - 1] > temp; j--) {
                nums[j] = nums[j - 1];
            }
            nums[j] = temp;
        }
    }
}