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;
    }
};