leetCode力扣3Sum

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本文介绍了一个经典的算法问题——寻找数组中三个数相加等于零的所有唯一组合，并提供了一个Java实现方案。该算法首先对数组进行排序，然后使用双指针技巧遍历数组以找到符合条件的三元组。

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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]
]

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

/**
 * Created by lxw, liwei4939@126.com on 2018/3/2.
 */
public class ThreeSum {
    public List<List<Integer>> threeSum(int[] nums){
        Arrays.sort(nums);
        List<List<Integer>> res = new LinkedList<>();
        for(int i = 0; i< nums.length-2; i++){
            if(i ==0 ||(i > 0 && nums[i] != nums[i-1])){
                int lo = i+1, hi = nums.length-1, sum = 0 - nums[i];
                while (lo < hi){
                    if(nums[lo] + nums[hi] == sum){
                        res.add(Arrays.asList(nums[i], nums[lo], nums[hi]));
                        while (lo < hi && nums[lo] == nums[lo+1]) lo++;
                        while (lo < hi && nums[hi] == nums[hi -1]) hi--;
                        lo++; hi--;
                    }
                    else if (nums[lo] + nums[hi] < sum) lo++;
                    else hi--;
                }
            }
        }
        return res;
    }
    public static void main(String[] args){
        int[] num = new int[]{-1,0,1,2,-1,-4};
        ThreeSum tmp = new ThreeSum();
        System.out.println(tmp.threeSum(num));
    }
}