[LeetCode 398] Random Pick Inde

Given an array of integers with possible duplicates, randomly output the index of a given target number. You can assume that the given target number must exist in the array.

Note:

• The array size can be very large. Solution that uses too much extra space will not pass the judge.

Example:

int[] nums = new int[] {1,2,3,3,3};
Solution solution = new Solution(nums);

// pick(3) should return either index 2, 3, or 4 randomly. Each index should have equal probability of returning.
solution.pick(3);

// pick(1) should return 0. Since in the array only nums[0] is equal to 1.
solution.pick(1);

Solution 1： With O(n) Space complexity

1. 用Reservoir Sampling的方法来找随机数。
2. Create an arraylist to store all index of target, and count total.
3. Then use random from [0,total), to equally pick any element in the array

List<Integer> list = new ArrayList<>();
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                total++;
                list.add(i);
            }
        }
        res = list.get(rand.nextInt(total)); //int in [0,total)

Solution 2： With O(1) Space complexity

1. For each number, if it is equal to target, count ++ and
2. (Reservoir Sampling) update by 1/total in the loop

random.nextInt(count)指在[0,count)之间随机地选取一个整数。代码中不一定要r==count-1，其实可以写成 r==0，因为只要保证当前概率是 1/count 即可

class Solution {
    int[] orginNum;

    public Solution(int[] nums) {
        orginNum = new int[nums.length];
        
        for (int i = 0; i < nums.length; i++) {
            orginNum[i] = nums[i];
        }
    }
    
    public int pick(int target) {
        int count = 0;
        int result = 0;
        Random rand = new Random ();
        
        for (int i = 0; i < orginNum.length; i++) {
            if (orginNum[i] == target) {
                count ++;
            
                if (rand.nextInt (count) == 0) {
                    result = i;
                }
            }
        }
        
        return result;
    }
}

/**
 * Your Solution object will be instantiated and called as such:
 * Solution obj = new Solution(nums);
 * int param_1 = obj.pick(target);
 */