【LeetCode算法练习（C++）】3Sum Closest

题目：
Given an array S of n integers, find three integers in S 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).

链接：3Sum Closest

解法：排序，用一对指针j,k搜索，三个数的和如果大于target，需要左移k,如果小于target，需要右移j，遍历i从最小到最大。时间O(n^2)

class Solution {public:    int threeSumClosest(vector<int>& nums, int target) {        int sum = 0, ans = INT_MAX / 2, len = nums.size();        sort(nums.begin(), nums.end());        for (int i = 0; i < len - 2; i++) {            int j = i + 1, k = len - 1;            while (j < k) {                int sum = nums[i] + nums[j] + nums[k];                ans = abs(sum - target) < abs(ans - target) ? sum : ans;                if (sum < target) {                    ++j;                } else if (sum > target) {                    --k;                } else {                    ++j;                    --k;                }            }        }        return ans;    }};

Runtime: 16 ms