Leetcode/各大家 -- 15. 3sum(3 poniter)

 15. 3Sum(3 ptr)

medium

https://leetcode.com/problems/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: 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]
]

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思路：Answer in the form of [..,i,j→ ,...,←k]
           outer: i for loop (0 - len-2) and skip duplicates
           inner: j = i+1, k= len-1 (while j<k), when sum equals 0, add results and skip dup

Complexity: O(n^2)

public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if(nums.length <= 2)return res;
        Arrays.sort(nums);
        for(int i = 0; i < nums.length - 2; i++){
            if(i > 0 && nums[i-1] == nums[i])continue;//skip dup
            int j = i + 1, k = nums.length - 1;//[..,i,j->, ..., <-k]
            while(j < k){
                int sum = nums[i] + nums[j] + nums[k];
                if(sum == 0){
                    res.add(Arrays.asList(nums[i],nums[j],nums[k]));
                    j++;
                    k--;
                    while(j < k && nums[j-1] == nums[j])j++;//skip duo
                    while(j < k && nums[k+1] == nums[k])k--;//skip dup
                }else if(sum < 0){
                    j++;
                }else{
                    k--;
                }
            }
        }
        return res;
    }