[LeetCode]Combination Sum
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Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.
The same repeated number may be chosen from C unlimited number of times.
Note:
- All numbers (including target) will be positive integers.
- Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
- The solution set must not contain duplicate combinations.
For example, given candidate set 2,3,6,7
and target 7
,
A solution set is: [7]
[2, 2, 3]
LeetCode Source
思路:DFS。或者是采用回溯法。特别注意要先对原数组排序。
class Solution {public: vector<vector<int> > combinationSum(vector<int> &candidates, int target) { vector<vector<int> >ret; vector<int> temp; sort(candidates.begin(),candidates.end()); for(int i=0;i<candidates.size();++i){ dfs(ret,candidates,i,target,temp); } return ret; } void dfs(vector<vector<int> >&ret,vector<int> candidates,int i,int target,vector<int> temp){ if(candidates[i]==target){ temp.push_back(candidates[i]); ret.push_back(temp); return; } if(candidates[i]>target){ return; } if(candidates[i]<target){ temp.push_back(candidates[i]); for(int j=0;j<candidates.size()-i;++j) dfs(ret,candidates,i+j,target-candidates[i],temp); } }};
AC了,但是发现运行时间长达435ms。分析后发现问题出在。
for(int j=0;j<candidates.size()-i;++j) dfs(ret,candidates,i+j,target-candidates[i],temp);
我们对明显不会有解(j>target-candidates[i])的结果进行了DFS。修改如下,时间减少到30ms。
class Solution {public: vector<vector<int> > combinationSum(vector<int> &candidates, int target) { vector<vector<int> >ret; vector<int> temp; sort(candidates.begin(),candidates.end()); for(int i=0;i<candidates.size();++i){ if(candidates[i]<=target) dfs(ret,candidates,i,target,temp); } return ret; } void dfs(vector<vector<int> >&ret,vector<int> candidates,int i,int target,vector<int> temp){ if(candidates[i]==target){ temp.push_back(candidates[i]); ret.push_back(temp); return; } if(candidates[i]<target){ temp.push_back(candidates[i]); for(int j=0;j<candidates.size()-i;++j) if(candidates[i+j]<=target-candidates[i]){ dfs(ret,candidates,i+j,target-candidates[i],temp); } } }};
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