LeetCode: 3Sum Closest

Problem:

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).

使用3Sum类似解法，时间复杂度O(nlogn+n^2)。

class Solution {public:    int threeSumClosest(vector<int> &num, int target) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        int nSize = num.size();        assert(nSize > 2);        sort(num.begin(), num.end());                int nMin = num[0] + num[1] + num[2];        int diff = abs(nMin - target);        for (int i = 0; i < nSize - 2; ++i)        {            int l = i + 1;            int r = nSize - 1;            int sum = 0;            while(l < r)            {                sum = num [i] + num[l] + num[r];                if (sum == target)  return sum;                if (abs(sum - target) < diff)                {                    nMin = sum;                    diff = abs(sum - target);                }                if (sum > target)                    --r;                else                    ++l;            }        }        return nMin;    }};