系統遞迴和自己用棧實現遞迴的比較

在dfs或者一些場合中，我們都會用到遞迴，提到遞迴大家都知道，是系統用系統棧協助我們來儲存資料。同樣我們也可以用自己的棧來實現遞迴。現在我再dfs（深度優先搜尋）的基礎上來淘淘下，系統遞迴和自己用棧實現遞迴在程式已耗用時間上的比較。我的程式是一道類似於求24點的dfs演算法的題

程式1（用stl的stack來實現dfs）

#include<iostream>#include <iomanip>#include<stdio.h>#include<cmath>#include<iomanip>#include<list>#include <map>#include <vector>#include <string>#include <algorithm>#include <sstream>#include <stack>#include<queue>#include<string.h>#include<set>using namespace std;int main(){int n;//cin>>n;scanf("%d",&n);for(int xx=0;xx<n;xx++){int num[5];int goal;for(int i=0;i<5;i++){//cin>>num[i];scanf("%d",num+i);}//cin>>goal;scanf("%d",&goal);int Max=-9999;clock_t start=clock();bool flag=false;stack< vector<int> >st;vector<int> init(num,num+5);st.push(init);while(!st.empty()){vector<int> to=st.top();st.pop();for(int i=0;i<to.size();i++){if(to[i]==goal){flag=true;break;}else if(to[i]<goal&&to[i]>Max){Max=to[i];}}for(int i=0;i<to.size();i++){for(int j=i+1;j<to.size();j++){vector<int> baseTmp;for(int k=0;k<to.size();k++){if(k!=i&&k!=j)baseTmp.push_back(to[k]);}vector<int> plusTmp(baseTmp);plusTmp.push_back(to[i]+to[j]);st.push(plusTmp);vector<int> minusTmp1(baseTmp);minusTmp1.push_back(to[i]-to[j]);st.push(minusTmp1);vector<int> minusTmp2(baseTmp);minusTmp2.push_back(to[j]-to[i]);st.push(minusTmp2);vector<int> multiplyTmp(baseTmp);multiplyTmp.push_back(to[i]*to[j]);st.push(multiplyTmp);if(to[i]==0||to[j]==0)continue;if(max(to[i],to[j])%min(to[i],to[j])==0){vector<int> divideTmp(baseTmp);divideTmp.push_back(max(to[i],to[j])/min(to[i],to[j]));st.push(divideTmp);}}}}if(flag)printf("%d\n",goal);else printf("%d\n",Max);clock_t end=clock();double Total_time = (double)(end-start) / CLOCKS_PER_SEC; printf( "%f seconds/n", Total_time); }}

測試結果：

程式2：（用系統遞迴實現dfs）

#include<iostream>#include <iomanip>#include<stdio.h>#include<cmath>#include<iomanip>#include<list>#include <map>#include <vector>#include <string>#include <algorithm>#include <sstream>#include <stack>#include<queue>#include<string.h>#include<set>using namespace std;int Max,goal;bool dfs(vector<long long> &to){for(int i=0;i<to.size();i++){if(to[i]==goal){return true;}else if(to[i]<goal&&to[i]>Max){Max=to[i];}}for(int i=0;i<to.size();i++){for(int j=i+1;j<to.size();j++){vector<long long> baseTmp;for(int k=0;k<to.size();k++){if(k!=i&&k!=j)baseTmp.push_back(to[k]);}vector<long long> plusTmp(baseTmp);plusTmp.push_back(to[i]+to[j]);if(dfs(plusTmp))return true;vector<long long> minusTmp1(baseTmp);minusTmp1.push_back(to[i]-to[j]);if(dfs(minusTmp1))return true;vector<long long> minusTmp2(baseTmp);minusTmp2.push_back(to[j]-to[i]);if(dfs(minusTmp2))return true;vector<long long> multiplyTmp(baseTmp);multiplyTmp.push_back(to[i]*to[j]);if(dfs(multiplyTmp))return true;if(to[i]==0||to[j]==0)continue;if(max(to[i],to[j])%min(to[i],to[j])==0){vector<long long> divideTmp(baseTmp);divideTmp.push_back(max(to[i],to[j])/min(to[i],to[j]));if(dfs(divideTmp))return true;}}}return false;}int main(){int n;//cin>>n;scanf("%d",&n);for(int xx=0;xx<n;xx++){vector<long long> num(5);goal;for(int i=0;i<5;i++){//cin>>num[i];scanf("%d",&num[i]);}//cin>>goal;scanf("%d",&goal);Max=-9999;clock_t start=clock();if(dfs(num))printf("%d\n",goal);else printf("%d\n",Max);clock_t end=clock();double Total_time = (double)(end-start) / CLOCKS_PER_SEC; printf( "%f seconds/n", Total_time); }}

結果：

可以看出系統遞迴比自己棧實現的遞迴已耗用時間少了四倍多，究其原因，可能系統用系統棧的開銷比我們自己的棧效能好很多，所以在遞迴比較多的時候，還是選用函數遞迴吧