3 Sum 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.

http://www.lintcode.com/en/problem/3-sum-closest/

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).

You may assume that each input would have exactly one solution.

O(n^2) time, O(1) extra space

Discussion

找最接近的那一组，二分行不通了，即使是排序以后也不行，因为可能错过最接近的那一组。先排序，再用两个首尾指针夹逼之。。

Solution

 class Solution {
public:    
    /**
     * @param numbers: Give an array numbers of n integer
     * @param target: An integer
     * @return: return the sum of the three integers, the sum closest target.
     */
    int threeSumClosest(vector<int> nums, int target) {
        // write your code here
        sort(nums.begin(), nums.end());
        vector<int>::iterator it;
        int result =0;
        int minGap = INT_MAX;
        for(it=nums.begin(); it<prev(nums.end(),2); it++) {
            auto start = next(it);
            auto end = prev(nums.end());
            while(start < end) {
                int sum = *it + *start + *end;
                int gap = abs(sum-target);
                if(gap < minGap) {
                    minGap = gap;
                    result = sum;
                }
                if(sum < target)
                    start++;
                else
                    end--;
            }
        }
        return result;
    }
};