LeetCode 3Sum Closest

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).

class Solution {public:    int threeSumClosest(vector<int> &num, int target) {        int closest = num[0]+num[1]+num[2], diff = abs(closest - target);        int i,j,k,s;        sort(num.begin(), num.end());        for(k = 0; k <= num.size() - 3; k++) {            i = k + 1;            j = num.size() - 1;            while(i < j) {                s = num[k]+num[i]+num[j];                if(abs(s - target) < diff) {                    closest = s;                    diff = abs(closest - target);                }                if(s == target) return target;                else if(s < target) i++;                else j--;            }        }        return closest;    }};

思路：解法时间复杂度O(n^2)，关键处思路同Two Sum。

先固定一个数，另外两个数使用Two Sum的算法来确定，然后这个固定的数取遍数组中所有值即可找出closest。