【数组】3Sum

题目：

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.

    For example, given array S = {-1 0 1 2 -1 -4},

    A solution set is:
    (-1, 0, 1)
    (-1, -1, 2)

思路：

设置两个指针，一个指头，一个指尾，用binary search找需要的元素。找到就加上，没有就去下一步。

/**
 * @param {number[]} nums
 * @return {number[][]}
 */
var threeSum = function(nums) {
    var n=nums.length,temp=0,res=[];
    if(nums.length<3){
        return [];
    }
    nums.sort(function(a,b){return a-b;});
    for(var i=0;i<n-2;i++){
        if(nums[i]>0){
            break;
        }
        for(var j=n-1;j>i+1;j--){
            temp=-(nums[i]+nums[j]);
            var l=i+1,r=j-1;
            if(nums[l]>temp||nums[r]<temp){
                break;
            }
            while(l<=r){
                var mid=l+Math.floor((r-l)/2);
                if(nums[mid]==temp){
                    var arr=[nums[i],nums[mid],nums[j]];
                    res[res.length]=arr;
                    break;
                }else if(nums[mid]<temp){
                    l=mid+1;
                }else{
                    r=mid-1;
                }
            }
        }
    }  

    return res;
};