1.5.cpp

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

char* replace1(char *c){
    if(c == NULL) return NULL;
    int len = strlen(c);
    if(len == 0) return NULL;
    int cnt = 0;
    for(int i=0; i<len; ++i)
    {
        if(c[i] == ' ')
            ++cnt;
    }
    char *cc = new char[len+2*cnt+1];
    int p = 0;
    for(int i=0; i<len; ++i)
    {
        if(c[i] == ' ')
        {
            cc[p] = '%';
            cc[p+1] = '2';
            cc[p+2] = '0';
            p += 3;
        }
        else
        {
            cc[p] = c[i];
            ++p;
        }
    }
    cc[p] = '\0';
    return cc;
}

void replace2(char *c){
    if(c == NULL) return;
    int len = strlen(c);
    if(len == 0) return;
    int cnt = 0;
    for(int i=0; i<len; ++i)
    {
        if(c[i] == ' ')
            ++cnt;
    }
    int p = len + 2*cnt;
    c[p--] = '\0';//the space must be allocated first.
    for(int i=len-1; i>=0; --i)
    {
        if(c[i] == ' ')
        {
            c[p] = '0';
            c[p-1] = '2';
            c[p-2] = '%';
            p -= 3;
        }
        else
        {
            c[p] = c[i];
            --p;
        }
    }
}
int main(){
    const int len = 100;
    char c[len] = "";
    cout<<replace1(c)<<endl;
    replace2(c);
    cout<<c<<endl;
    return 0;
}