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).

分析：基本思想和3Sum差不多，不过本题要求求解与target最接近的三数之和。同理，先对数组进行排序，然后两边夹逼。在设置初始最接近的数字时，设为int型最大值。

运行时间：15ms

class Solution {
public:
    int threeSumClosest(vector<int>& nums, int target) {
        //if(nums.size() < 3) return 0;
        if(nums.size() == 3) return nums[0] + nums[1] + nums[2];
        sort(nums.begin(), nums.end());
        int closest = pow(2, 31) - 1, result = 0;
        
        for(int a = 0; a < nums.size() - 2; a++){
            int b = a + 1, c = nums.size() - 1;
            while(b < c){
                int diff = nums[a] + nums[b] + nums[c] - target;
                if(diff < 0) b++;
                else if(diff > 0) c--;
                else return target;
                
                if(abs(diff) < closest){
                    result = diff + target;
                    closest = abs(diff);
                }
            }
        }
        return result;
    }
};