String replace space

subject : One string space is enough , Replace each space in the string with "%20", Such as "He ll o" Replace with "He%20ll%20o"

Ideas : First traverse the string , Get the number of spaces , To get the length of the new string . Traverse the string from the end of the string , Copy characters one by one , Replace with... When a space is encountered %20.

#include <iostream>

void trimBlank(char arr[], int length)
{
    if (arr == NULL || length <= 0) {
        return;
    }
    
    int index = 0;
    int blankNumber = 0;
    while (arr[index]!= '\0') {
        if (arr[index] == ' ') {
            blankNumber++;
        }
        index++;
    }
    
    int replaceIndex = length + blankNumber * 2;
    int i = length;
    while (i >=0 && replaceIndex > i) {
        if (arr[i] == ' ') {
            arr[replaceIndex--] = '0';
            arr[replaceIndex--] = '2';
            arr[replaceIndex--] = '%';
        } else {
            arr[replaceIndex--] = arr[i];
        }
        i--;
    }
}

int main(int argc, const char * argv[]) {
    char a[20] = {'h', ' ', 'e','l','l', ' ', 'o'};
    trimBlank(a, 7);
    std::cout<<a<<std::endl;
    return 0;
}