【leetcode】URL化

char* replaceSpaces(char* S, int length){
    char* s = (char*)calloc(length*3+1,sizeof(char));
    int n = 0;
    for (int i=0; i<length; i++)
    {
        if (S[i] == ' ')
        {
            s[n++] = '%';
            s[n++] = '2';
            s[n++] = '0';
        }
        else
            s[n++] = S[i];
    }
    return s;
}