LeetCode #16 3Sum Closest

Description

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).

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Analysis

题目难度为：Medium
本题思路，n^3的遍历所有的情况。我们可以进行一定的剪枝，对于第一层循环for (i : nums)，对于另j = i+1，k = nums.size()，当i+j+k>target时，–k；i+j+k

Code（c++)

class Solution {public:    int threeSumClosest(vector<int>& nums, int target) {        if(nums.size() < 3) return 0;        int closest = nums[0]+nums[1]+nums[2];        sort(nums.begin(), nums.end());        for(int first = 0 ; first < nums.size()-2 ; ++first) {            if(first > 0 && nums[first] == nums[first-1]) continue;            int second = first+1;            int third = nums.size()-1;                        while(second < third) {                int curSum = nums[first]+nums[second]+nums[third];                if(curSum == target) return curSum;                if(abs(target-curSum)<abs(target-closest)) {                    closest = curSum;                }                if(curSum > target) {                    --third;                } else {                    ++second;                }            }        }        return closest;    }};