Replace All Digits with Characters

给一个string, 里面有偶数位的字符和奇数位的数字组成, 给一个方法decode这个string, 是用字符后的数字位个字符代替数字位. 求decoded string.

class Solution {
public:
    string replaceDigits(string s) {
        int n = s.size();
        for(int i = 1; i < n; i+=2){
            char c = (char)(s[i - 1] + (s[i] - '0'));
            s[i] = c;
        }
        return s;
    }
};