leetcode 18.4Sum
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1.题目
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
- Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
- The solution set must not contain duplicate quadruplets.
For example, given array S = {1 0 -1 0 -2 2}, and target = 0. A solution set is: (-1, 0, 0, 1) (-2, -1, 1, 2) (-2, 0, 0, 2)
从一个数组中找到四个数,使这四个数的和为target,返回这四个数。
2.思路
思路还是跟3sum 一样,先贴上我的代码,有很多的地方可以优化。
class Solution { //192mspublic: vector<vector<int>> fourSum(vector<int>& nums, int target) { vector<vector<int>> ans; int len=nums.size(); if(len<4) return {}; sort(nums.begin(),nums.end()); for(int i=0;i<len-3;) { for(int j=i+1;j<len-2;) { int head=j+1,tail=len-1; while(head<tail) { if(nums[head]+nums[tail]+nums[i]+nums[j] == target) { ans.push_back(vector<int>{nums[i],nums[j],nums[head],nums[tail]}); do{tail--;} while(nums[tail] == nums[tail+1] && head<tail); do{head++;} while(nums[head] == nums[head-1] && head<tail); } else if(nums[head]+nums[tail]+nums[i]+nums[j] > target) { do{tail--;} while(nums[tail] == nums[tail+1] && head<tail); } else { do{head++;} while(nums[head] == nums[head-1] && head<tail); } } do{j++;} while(nums[j] == nums[j-1] && j<len-2); } do{i++;} while(nums[i] == nums[i-1] && i<len-3); } return ans; }};再贴上disguss中的高票答案,https://leetcode.com/discuss/67417/my-16ms-c-code
class Solution { //16mspublic: vector<vector<int>> fourSum(vector<int>& nums, int target) { vector<vector<int>> total; int n = nums.size(); if(n<4) return total; sort(nums.begin(),nums.end()); for(int i=0;i<n-3;i++) { if(i>0&&nums[i]==nums[i-1]) continue; //这里,相等的数不重复检索 if(nums[i]+nums[i+1]+nums[i+2]+nums[i+3]>target) break; //如果初始值都大于target,那么之后的肯定找不到了 if(nums[i]+nums[n-3]+nums[n-2]+nums[n-1]<target) continue;//在i固定的情况下,加上最大的三个数都小于target for(int j=i+1;j<n-2;j++) { if(j>i+1&&nums[j]==nums[j-1]) continue; if(nums[i]+nums[j]+nums[j+1]+nums[j+2]>target) break; if(nums[i]+nums[j]+nums[n-2]+nums[n-1]<target) continue; int left=j+1,right=n-1; while(left<right){ int sum=nums[left]+nums[right]+nums[i]+nums[j]; if(sum<target) left++; else if(sum>target) right--; else{ total.push_back(vector<int>{nums[i],nums[j],nums[left],nums[right]}); do{left++;}while(nums[left]==nums[left-1]&&left<right); do{right--;}while(nums[right]==nums[right+1]&&left<right); } } } } return total; }};
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