（栈）leetcode 394

class Solution {
public:
    string decodeString(string s) {
        stack<int> nums;
        stack<string> ch;
        int num = 0;
        string str = "";
        for(auto c:s){
            if(isdigit(c))
                num = num*10 + (c-‘0‘);
            else if(isalpha(c))
                str += c;
            else if(c==‘[‘){
                ch.push(str);
                nums.push(num);
                str = "";
                num = 0;
            }
            else{
                string tmp = str;
                for(int i=0; i<nums.top()-1; ++i)
                    //本身一次已经在内，故nums.top()-1
                    str += tmp;
                //这里字符相加的顺序不能颠倒
                str = ch.top()+str;
                nums.pop();
                ch.pop();
            }
        }
        return str;
    }
};