1-4.cpp

#include <iostream>
#include <string>
using namespace std;

void replace_per(string &s)
{
    int space_num = 0;
    for (auto it = s.begin(); it != s.end(); it++) {
        if (*it == ' ') {
            space_num++;
        }
    }
    int old_size = s.size();
    int new_size = s.size() + 2 * space_num;
    s.resize(new_size);
    
    auto new_end = s.end() - 1;
    auto old_end = s.begin() + old_size - 1;
    
    while (old_end != s.begin()) {
        if (*old_end != ' ') {
            //not space
            *new_end = *old_end;
        } else {
            //space
            *new_end = '0';
            *(--new_end) = '2';
            *(--new_end) = '%';
        }
        old_end--;
        new_end--;
    }
    
    return;
}

/*
    Input:  "Mr John Smith"
    Output: "Mr%20John%20Smith"
*/
int main(int argc, char *argv[]) {
    string s = "Mr John Smith";
    replace_per(s);
    cout << s << endl;
    
    return 0;
}

void replace_per(string& s)
{
size_t pos ;
while ( pos = s.find(" ") != -1)
s.replace( pos, pos + 1, "%20");
return ;
}
Why no?