#include <stdio.h>
#include <string.h>
#include <stdlib.h>

void reverse(char* str) {
    int forward = 0;
    int backward = strlen(str) - 1;

    while (forward < backward) {
        char temp = str[forward];
        str[forward++] = str[backward];
        str[backward--] = temp;
    }
}

void right_align(char *str) {
    reverse(str);

    while (*str == ' ') {
        str += 1;
    }

    reverse(str);
}

void replace_spaces(char *str, size_t length) {
    const char * end = str + length;
    const size_t token_length = strlen("%20");
    char * out = str;

    right_align(str);

    while (*str == ' ') {
        str += 1;
    }

    while (str < end) {
        if (*str != ' ') {
            *out = *str;
            out += 1;
        } else {
            strncpy(out, "%20", token_length);
            out += token_length;
        }
        str += 1;
    }
}

int main(int argc, char *argv[]) {
    if (argc == 3) {
        replace_spaces(argv[1], atoi(argv[2]));
        printf("%s\n", argv[1]);
    } else {
        printf("usage: %s <string> <length>\n", argv[0]);
    }
}