【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).

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) 。

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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