思路:兩種方法都是利用遞歸回溯,第二方法在第一種方法的基礎上對原始數據先進行排序,這樣可以剪枝,加快計算速度。第一種方法在LeetCode上測試運行的時間是24ms,第二種方法運行時間爲16ms。
方法一:
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
class Solution {
private:
//res保存滿足題意的結果
vector<vector<int>> res;
//保存當前的一種方案
vector<int> method;
public:
void getCombinationSum(int start, int sum, int target, vector<int> &candidates)
{
//因爲canditates經過排序,所以如果sum>target
if (sum > target)
{
return;
}
//如果滿足題意則保存結果
else if (sum == target)
{
res.push_back(method);
return;
}
else
{
for(int i=start;i<candidates.size();++i)
{
//芳第i件物品
method.push_back(candidates[i]);
getCombinationSum(i, sum+candidates[i], target, candidates);
//回溯,去除第i件物品,然後繼續放第i+1個數
method.pop_back();
}
}
}
vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
int sum = 0;
getCombinationSum(0, 0, target, candidates);
return res;
}
};
int main()
{
Solution s;
vector<int> canditates;
canditates.push_back(2);
canditates.push_back(7);
canditates.push_back(3);
canditates.push_back(6);
vector<vector<int>> res = s.combinationSum(canditates, 7);
//打印結果
for (vector<vector<int>>::iterator ita=res.begin();ita!=res.end();++ita)
{
for (vector<int>::iterator itb=ita->begin();itb!=ita->end();++itb)
{
cout<<*itb<<'\t';
}
cout<<endl;
}
cout<<endl;
return 0;
}
方法二:
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
class Solution {
private:
//res保存滿足題意的結果
vector<vector<int>> res;
//保存當前的一種方案
vector<int> method;
public:
void getCombinationSum(int start, int sum, int target, vector<int> &candidates)
{
for(int i=start;i<candidates.size();++i)
{
//放第i個數,如果第i個數放進去之後超過了target,則剪枝(直接返回),因爲candidates經過排序,換成後面的數放進去的話肯定也會超過target
if (sum+candidates[i] > target)
{
return;
}
//如果滿足則保存結果
else if (sum+candidates[i] == target)
{
method.push_back(candidates[i]);
res.push_back(method);
method.pop_back();
return;
}
//如果放第i個數後不超過target,則將第i個數放入
else
{
method.push_back(candidates[i]);
getCombinationSum(i, sum+candidates[i], target, candidates);
//回溯,去除第i個數,然後繼續放第i+1個數
method.pop_back();
}
}
}
vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
int sum = 0;
//排序
sort(candidates.begin(), candidates.end());
getCombinationSum(0, 0, target, candidates);
return res;
}
};
int main()
{
Solution s;
vector<int> canditates;
canditates.push_back(2);
canditates.push_back(7);
canditates.push_back(3);
canditates.push_back(6);
vector<vector<int>> res = s.combinationSum(canditates, 7);
for (vector<vector<int>>::iterator ita=res.begin();ita!=res.end();++ita)
{
for (vector<int>::iterator itb=ita->begin();itb!=ita->end();++itb)
{
cout<<*itb<<'\t';
}
cout<<endl;
}
cout<<endl;
return 0;
}