Closest 3 Sum

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).

代码：

   int threeSumClosest(vector<int> &num, int target) {
        // sort(num.begin(),num.end()); sort this array if unsorted
        int n = num.size();
        int cl = num[0]+num[1]+num.back();
        int sum;
        for(int i=0;i<n-2;++i){
            int j=i+1;
            int k=n-1;
            while(j<k){
                sum = num[i]+num[j]+num[k];
                if(abs(sum-target)<abs(cl-target))  cl=sum;
                if(sum==target)                     return target;
                if(sum<target){
                        sum = num[i]+num[++j]+num[k];
                }
                else{
                        sum = num[i]+num[j]+num[--k];
                }
            }
        }
        return cl;
    }