LeetCode关于数组中求和的题型

15. 3Sum：三数之和为0的组合

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]
]思路：循环法，先给数组排序->[-4,-1,-1,0,1,2],其中i,j,k三数的下标，i先为最左边的一个数，则i<n-3,此时可以根据求两数之和的方法来进行求解；对一个有序的数组求两数之和为定值，最简单的方法为j,k分别为数组的两端，主键向中间靠拢；具体代码如下：

import java.util.*;
public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> lists = new ArrayList<>();
        if(nums==null||nums.length==0)
            return lists;
        Arrays.sort(nums);
        int n = nums.length;
        int i=0;
        while(i<=n-3){
            int j=i+1;
            int k = n-1;
            while(j<k){
                if(nums[i]+nums[j]+nums[k]==0){
                    ArrayList<Integer> list = new ArrayList<Integer>();
                    list.add(nums[i]);
                    list.add(nums[j]);
                    list.add(nums[k]);
                    lists.add(list);
                    while(j<k&&nums[j]==nums[j+1])
                        j++;
                    while(k>j&&nums[k]==nums[k-1])
                        k--;
                    j++;
                    k--;
                }else if(nums[i]+nums[j]+nums[k]<0){
                    j++;
                }else{
                    k--;
                }
            }
            while(i<n-1&&nums[i]==nums[i+1])
                i++;
            i++;
        }
        return lists;
    }
}