[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.

标签：

　　Array

体会：

1. 这道题最直接的想法肯定是想 A³_n的那种解法，就是遍历所有可能的三个数的排列，然后找结果。可是这样就是A3n = n(n-1)(n-2) = O(n³)的解法，会exceed time limit。

2. 然后就是低于3次的解法了。

 

class Solution {
    public:
        int threeSumClosest(vector<int> &num, int target) {
            // we don't want to change the original vector probably
            // how to declare vector in C++
            vector<int> v(num.begin(), num.end());
            
            int result = 0;
            int size = v.size();
            if (size <= 3) {
                for (int i = 0; i < size; i++){
                    result += v[i];    
                }
                return result;
            }
            std::sort(v.begin(), v.end());

            result = v[0] + v[1] + v[2];
            int sum = 0;
            for (int i = 0; i < size - 2; i++) {
                int j = i + 1;
                int k = size - 1;
                while (j < k) {
                    sum = v[i] + v[j] + v[k];
                    if (std::abs(target - sum) < std::abs(target - result)) {
                        result = sum;
                        if (result == target) {
                            return result;
                        }
                    }
                    if (sum > target) {
                        k--;
                    } else {
                        j++;
                    }
                }
            }
            return result;
        }
};