015. 3sum

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

即在序列中找出和为0的三个数。

public class Solution {
  public List<List<Integer>> threeSum(int[] nums) {
    Arrays.sort(nums);
    List result = new ArrayList<List<Integer>>();
    
    if (nums == null || nums.length < 3) return result;  
    
    
    for(int i = 0; i < nums.length - 2 && nums[i] <= 0 ; i++) {
        if (i > 0 && nums[i] == nums[i-1]) continue;
        int left = i + 1;
        int right = nums.length - 1;
        while (right > left) {
            int tmp = nums[i] + nums[left] + nums[right];
            if (tmp > 0) while(left < right && nums[--right] == nums[right+1]);
            if (tmp < 0) while(left < right && nums[++left] == nums[left-1]);
            if (tmp == 0) {
                List l = new ArrayList<Integer>();
                l.add(nums[i]);
                l.add(nums[left]);
                l.add(nums[right]);
                while(left < right && nums[++left] == nums[left-1]);
                while(left < right && nums[--right] == nums[right+1]);
                result.add(l);
            }
        }
    }
    return result;
  }
}