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).

class Solution {
public:
    static bool compare_func(const int d1, const int d2) {  
        return d1 < d2;  
    }
    
    int threeSumClosest(vector &num, int target) {
        int i, j, k;  
        int diff, sum;
        sort(num.begin(), num.end(), compare_func);  
        int closest = num[0] + num[1] + num[2];
        int diffmin = abs(target - closest);
        for(i = 0; i < num.size(); i++) {  
            for(j = i + 1, k = num.size() - 1; j < k ; ) { 
                diff = abs(target - num[i] - num[j] - num[k]); 
                if(diff < diffmin) {
                    diffmin = diff;
                    closest = num[i] + num[j] + num[k];
                }
                if(num[i] + num[j] + num[k] < target) j++;
                else if(num[i] + num[j] + num[k] > target) k--;
                else return target;
            }
        }
        return closest;
    }
};