[leetcode 16] 3Sum Closest

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).

思路：同3Sum， 排序之后，夹逼

class Solution {public:    int threeSumClosest(vector<int> &num, int target) {        int res;        if (num.size() < 3) {            return res;        }        int min_gap = INT_MAX;  //记录最小的差值        sort(num.begin(), num.end());        auto last = num.end();        for (auto i = num.begin(); i < last - 2; i++) {            auto j = i + 1;            auto k = last - 1;            while (j < k) {                int sum = *i + *j + *k;                int gap = abs(sum - target);                if (gap < min_gap) {                    res = sum;                    min_gap = gap;                }                if (sum > target) {                    k--;                } else {                    j++;                }            }        }        return res;    }};