Leetcode_3Sum Closest

题目：

Given an array S ofn integers, find three integers inS such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

For example, given array S = {-1 2 1 -4}, and target = 1.

    The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

和3sum相似。

最“简单”即最容易想到的方法肯定是，所有数组合一次，n(n-1)(n-2)个组合，这样就是在三层数量基本都是n的数据上进行处理。将数据排序后，以下方法是线性的，在一层数据上处理。视觉上，可以从上述角度去理解这个方法的优化之处。

代码如下：

class Solution {public:    int threeSumClosest(vector<int>& nums, int target) {        int closet = nums[0] + nums[1] + nums[2];        int diff = abs(closet-target);        sort(nums.begin(), nums.end());        for(int i = 0; i < nums.size()-2; i++) {        int left = i+1, right = nums.size()-1;        while(left < right) {        int sum = nums[i] + nums[left] + nums[right];        if(diff > abs(sum - target)) {        diff = abs(sum - target);        closet = sum;        }        if(sum < target) {        left++;        }        else {        right--;        }        }        }        return closet;    }};