LeetCode 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).

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