[LeetCode] 3Sum Closest

[Problem]

Given an array S of n integers, find threeintegers in S such that the sum is closest to a givennumber, target. Return the sum of the three integers. You mayassume 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).

[Analysis]
先对数据按照从小到大排序，然后从头到尾遍历各组合，注意剪枝。

[Solution]

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
       
        // sort data
        sort(num.begin(), num.end());
        int len = num.size();

        // initial result
        int res = num[0] + num[1] + num[2];

        // closest
        int min = target > res ? target - res : res - target;

        // search
        for(int i = 0; i < len - 2; ++i){
                for(int j = i + 1; j < len - 1; ++j){
                        for(int k = j + 1; k < len; ++k){
                                // 3sum
                                int sum = num[i] + num[j] + num[k];

                                // closest
                                int clo = target > sum ? target - sum : sum - target;
                                if(clo < min){
                                        res = sum;
                                        min = clo;
                                }      

                                // cut 
                                if(sum > target)break;
                        }      
                }      
        }      
        return res;
    }
};

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