剑指 Offer 05. 替换空格

请实现一个函数，把字符串 s 中的每个空格替换成"%20"。

class Solution {
public:
    string replaceSpace(string s) {
        int len = s.size();
        int count = 0;
        //统计空格数量
        for (char& c : s) {
            if (c == ' ') count++;
        }
        //定义s_copy长度
        int s_len = len + count * 2;
        std::string s_copy;
        s_copy.resize(s_len);
        //遍历修改
        int i = len-1;
        int j = s_len-1;
        while (i>=0) {
            if (s[i] == ' ') {
                s_copy[j] = '0';
                s_copy[j - 1] = '2';
                s_copy[j - 2] = '%';
                j = j - 3;
            }
            else {
                s_copy[j] = s[i];
                j = j - 1;
            }
            i = i - 1;
        }
        return s_copy;
    }
};

已经造好的轮子：

return s.replace(" ","%20");