【LeetCode】16. 3Sum Closest

题目

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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思路

对数组排序，先固定一个数，然后设置两个指针分别指向头和尾，计算和是否比当前最优解还要接近目标值。然后判断和与目标值的大小，进行指针的移动。复杂度O(n2)。

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代码

class Solution {public:    int sum(vector<int>& num){        int s=0;        for (int n:num){            s += n;        }        return s;    }    int threeSumClosest(vector<int>& nums, int target) {         int i=0,j,k;         int n = nums.size();         if (n<=3) return sum(nums);         sort(nums.begin(),nums.end());         int ans = nums[0]+nums[1]+nums[2];         for (int i=0;i<n-2;++i){             j=i+1;k=n-1;             while (j<k){                 int tsum = nums[i]+nums[j]+nums[k];                 if (abs(target-tsum)<abs(target-ans)){                     ans = tsum;                     if (ans==target) return ans;                 }                 tsum>target?k--:j++;             }         }         return ans;    }};

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