Cacher is the code snippet organizer for pro developers

// http://learn.hackerearth.com/question/168/in-a-string-replace-all-occurrence-of-the-given-pattern-to-x/
/*
In a string replace all occurrence of the given pattern to 'X'

Replace all occurrence of the given pattern to 'X'.
For example, given that the pattern="abc", replace "abcdeffdfegabcabc" with "XdeffdfegX". Note that
multiple occurrences of abc's that are contiguous will be replaced with only one 'X'.
*/


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

// ================================ C version ================================

bool is_match(char *s, const char *p) {
    while(*p) { // gist, cannot write (*p && *s)
        if(*s != *p) return false;
        s++;
        p++;
    }
    return true;
}
 
char* replace_patterns(char *s, const char *patt) {
    if(!s || *s == '\0' || !patt || *patt == '\0') return NULL;
    int end = 0, i = 0, plen = strlen(patt);
    while(s[i] != '\0') {
        bool match = false;
        while(is_match(s+i, patt)) {
            match = true;   
            i += plen;
        }
        if(match) s[end++] = 'X';
        else s[end++] = s[i++];
    }
    s[end] = '\0'; // necessary
    return s;
}

int main()
{
    char s[] = "abcdeffdfegabcabc";
    // char *s = "abcdeffdfegabcabc"; // wrong!!! segment fault!!!
    cout << replace_patterns(s, "abc");
}

// ================================ C++ version ================================

bool is_match(string s, const string& patt) {
    int ns = s.size(), np = patt.size();
    if(ns < np) return false;
    for(int i=0; i<np; i++)
      if(s[i] != patt[i]) return false;
    return true;
}

string replace_pattern(string& s, const string& p) {
    if(s.empty()) return "";
    if(p.empty()) return s;
    int ns = s.size(), np = p.size();
    if(ns < np) return s;
    
    int tail = 0;
    int i = 0;
    while(i < ns) {
        // int j = i, k = 0; // no need to have j and k
        bool match = false;
        while(is_match(s.substr(i), p)) {
            match = true;
            i += np;
        }
        if(match) s[tail++] = 'X';
        else s[tail++] = s[i++];
    }
    return s.substr(0, tail);
}

int main() {
    string s = "abcabcxaX";
    cout << replace_pattern(s, "abc");
}