Uva536 binary tree reconstruction tree recovery

Enter the sequence of preorder traversal and middle order traversal of a binary tree , Output its post order traversal sequence .

#include<bits/stdc++.h>
using namespace std;

string preorder,inorder;

void postorder(string pre,string in){
    
    if(pre.size() <= 0){
    
        return ;
    }
    int len = 0;
    
    len = in.find(pre[0]);
    postorder(pre.substr(1,len),in.substr(0,len));
    postorder(pre.substr(len+1).in.substr(len+1));
    cout<<pre[0];
}

int mian(){
    
    while(cin>>preorder>>inorder){
    
        postorder(preorder,inorder);
        cout<<endl;
    }
    
    return 0;
}

#include<bits/stdc++.h>
using namespace std;

string preorder,inorder;

void postorder(int l1,int l2,int n){
    
    if(n <= 0){
    
        return ;
    }
    int len = 0;
    
    int len = inorder.find(preorder[l1]) - l2;
    postorder(l1+1,l2,len);
    postorder(l1+len+1,l2+len+1,n-len-1);
    cout<<preorder[l1];
}

int mian(){
    
    while(cin>>preorder>>inorder){
    
    	int len = preorder.size();
        postorder(0,0,len);// Preamble start subscript , Middle order start subscript , length 
        cout<<endl;
    }
    
    return 0;
}