剑指offer(2)：替换空格

请实现一个函数，将一个字符串中的空格替换成“%20”。例如，当字符串为We Are Happy.则经过替换之后的字符串为We%20Are%20Happy。

class Solution {
public:
    void replaceSpace(char *str,int length) {
        int newlength = 0, blank_number = 0, str_len = 0, tail = 0;
        for(str_len = 0; str[str_len] != '\0'; str_len++)
        {
            if(str[str_len] == ' ')
                blank_number += 2;
        }
        
        newlength = str_len + blank_number;
        if(newlength > length)
            return;
        
        str[newlength] = '\0';
        tail = newlength - 1;
        for( auto i = str_len - 1; i >= 0; i--)
        {
            if( str[i] == ' ')
            {
                str[tail--] = '0';
                str[tail--] = '2';
                str[tail--] = '%';
            }
            else
                str[tail--] = str[i];
        }
    }
};