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: 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]]

2sum的升级版 想办法转化为2sum就可以了

第二遍 做出来了 

答案有几点值得学习 

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