class Solution {
public:
    string replaceSpace(string s) {
        int pos =0;
        while(s[pos]!= '\0')
        {
           if(s[pos]==' ')
           { 
                s.erase(pos,1);
                s.insert(pos,"%20");
           }
               pos++;
        }
        return s;
    }
};

class Solution {
public:
    string replaceSpace(string s) {
        int count = 0; // 统计空格的个数
        int sOldSize = s.size();
        for (int i = 0; i < s.size(); i++) {
            if (s[i] == ' ') {
                count++;
            }
        }
        // 扩充字符串s的大小，也就是每个空格替换成"%20"之后的大小
        s.resize(s.size() + count * 2);
        int sNewSize = s.size();
        // 从后先前将空格替换为"%20"
        for (int right = sNewSize - 1, left = sOldSize - 1; left < right; right--, left--) {
            if (s[left] != ' ') {
                s[right] = s[left];
            } else {
                s[right] = '0';
                s[right - 1] = '2';
                s[right - 2] = '%';
                right -= 2;
            }
        }
        return s;
    }
};

class Solution {
public:
    string replaceSpace(string s) {
        for(int i=0 ; i<s.size() ;i++)
        {
            if(s[i] == ' ')
            {
                s.erase(i,1);
                s.insert(i,"%20");
            }
        }
        return s;
    }
};