3Sum Closest

来源：Leetcode

原帖：http://oj.leetcode.com/problems/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).

Solution: Similar to 3Sum, taking O(n^2) time complexity.

代码：

 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)) { // trick: overflow  
                     res = threesum;  
                 }  
             }  
         }  
         return res;  
     }  
 };