3Sum Closest

06/25/2016 Array Two Pointers

Question
Solution

Question

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).

Solution

Result: Accepted Time: 28ms

Here should be some explanations.

class Solution {
    inline int iabs(int a)
    {
        return a > 0 ? a : -a;
    }
public:
    int threeSumClosest(vector<int>& nums, int target) {
        sort(nums.begin(),nums.end());
        const int len = nums.size();
        int ret = 0x3fffffff;
        for(int i = 0; i < len - 2; i++)
        {
            int low = i + 1, up = len - 1, diff = nums[i] - target;
            while(low < up)
            {
                int tmp = nums[low] + nums[up] + diff ;
                if(tmp > 0)
                    up --;
                else
                    low ++;
                if(iabs(tmp) < iabs(ret-target))
                    ret = tmp + target;
            }
        }
        return ret;
    }
};

Complexity Analytics

Time Complexity: $O(n^2$
Space Complexity: $O(1)$