在C ++中做了一个递归函数。 现在要求进行“尾递归”功能

Question

Here is a recursive function I came up with in order to fulfill the requirements of duplicating each letter in a string (ie "abc" to "aabbcc"):

#include <string>
#include <iostream>

using namespace std;

string repeater(string str){
    if (str == ""){
        return "";
    }
    else{
        return str.substr(0,1) + str.substr(0,1) + repeater(str.substr(1));
    }
}

int main(){
    string test = "llama";
    cout << "repeater(\"" << test << "\") returned: " << repeater(test) << endl;
}

Now I'm asked to make a "tail-recursive" function that does the same thing. I'm reading up on it, and my text gives the following example of a "tail-recursive" function:

int factorial(int n) {
if (n == 0)
return 1;
else
return n * factorial(n – 1);
}

This looks identical to the method I used to create my own. I need to hand in 2 versions of recursive functions. Is mine already a "tail-recursive" function? If so, how would I make a forward recursive function (or, if the opposite is the case, vice-versa)?

http://ideone.com/JrtRsE

Answer 1

I suggest the following:

string tail_repeater(string left, string right){
    if (right == ""){
       return left;
    }
    return tail_repeater(left + right.substr(0,1) + right.substr(0,1), right.substr(1));
}

string repeater(string str){
   tail_repeater("", str);
}