3SumClosest

3SumClosest

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)

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        int res = INT_MAX;
        int N = num.size();
        sort(num.begin(), num.end());
        for (int i = 0; i < N-2; ++i) {
            int l = i + 1, r = N - 1;
            while (l < r) {
                int threesum = num[i] + num[l] + num[r];
                if (threesum == target) return target;
                else if (threesum < target) l++;
                else r--;
                if (res == INT_MAX || abs(threesum - target) < abs(res - target)) // overflow tricky
                    res = threesum;
            }
        }
        return res;
    }
};