15. 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.

Note

Elements in a triplet (a,b,c) must be in non-descending order. (ie, a ≤ b ≤ c)The solution set must not contain duplicate triplets.

For example, given array S = {-1 0 1 2 -1 -4}, A solution set is: (-1, 0, 1) (-1, -1, 2)

code

public class Solution {    public List<List<Integer>> threeSum(int[] nums) {        List<List<Integer>> rst = new ArrayList<List<Integer>>();        if(nums == null || nums.length < 3) {            return rst;        }        Arrays.sort(nums);        for (int i = 0; i < nums.length - 2; i++) {            if (i != 0 && nums[i] == nums[i - 1]) {                continue; // to skip duplicate numbers; e.g [0,0,0,0]            }            int left = i + 1;            int right = nums.length - 1;            while (left < right) {                int sum = nums[left] + nums[right] + nums[i];                if (sum == 0) {                    ArrayList<Integer> tmp = new ArrayList<Integer>();                    tmp.add(nums[i]);                    tmp.add(nums[left]);                    tmp.add(nums[right]);                    rst.add(tmp);                    left++;                    right--;                    while (left < right && nums[left] == nums[left - 1]) { // to skip duplicates                        left++;                    }                    while (left < right && nums[right] == nums[right + 1]) { // to skip duplicates                        right--;                    }                } else if (sum < 0) {                    left++;                } else {                    right--;                }            }        }        return rst;    }}