LeetCode（15）——3Sum

题目：

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

Note:

The solution set must not contain duplicate triplets.

Example:

Given array nums = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]

AC：

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> result = new LinkedList<>();
    	Arrays.sort(nums);
        if (null == nums || nums.length < 3 || nums[nums.length - 1] < 0) {
            return result;
        }
    	for (int x = 0; x < nums.length - 2 && nums[x] <= 0; x++) {

            if (x == 0 || nums[x] != nums[x - 1]) {
            	int y = x + 1;
                int z = nums.length - 1;
                int target = -nums[x];
                while (y < z) {
                    if (nums[y] + nums[z] == target) {
                        result.add(Arrays.asList(nums[x], nums[y], nums[z]));
                        y++;
                        z--;
                        while (y < z && nums[y] == nums[y - 1]) {
                            y++;
                        }
                        
                        while (y < z && nums[z] == nums[z + 1]) {
                            z--;
                        }
                    }
                    else if (nums[y] + nums[z] < target) {
                        y++;
                    }
                    else {
                        z--;
                    }
                }
            }
    	}
    	
    	return result;
    }
}