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.

public class Solution {public List<List<Integer>> threeSum(int[] nums) {Arrays.sort(nums);//sort进行排序List<List<Integer>> list = new ArrayList<List<Integer>>();for(int i = 0; i < nums.length-2; i++) {if(i > 0 && (nums[i] == nums[i-1]))continue;//避免重复for(int j = i+1, k = nums.length-1; j < k;) {int sum = nums[i] + nums[j] + nums[k];if(sum == 0) {list.add(Arrays.asList(nums[i],nums[j],nums[k]));j++;k--;while((j < k) && (nums[j] == nums[j-1]))j++;//避免重复while((j < k) && (nums[k] == nums[k+1]))k--;//避免重复}else if (sum > 0)k--;//结果大于0，大数往里缩elsej++;//结果小于0，小数往里缩}}return list;}}

//大致思路就是先排序，然后第一个数从0到num.length-2遍历，后面俩数初始放在最前面和最后面，然后慢慢的向里面靠近。此算法打败了92.68%的人。

//发现很多算法题都是两边入手往里，或者找到中间值向外扩。