394、字符串解码

示例 1：

输入：s = "3[a]2[bc]"
输出："aaabcbc"

示例 2：

输入：s = "3[a2[c]]"
输出："accaccacc"

public static String decodeString(String s) {
    Stack<Integer> numStack = new Stack<>();       //数字栈
    Stack<String> strStack = new Stack<>();       //字符串栈
    StringBuilder res = new StringBuilder();
    int num = 0; //重复次数
    //遍历每个字符
    for (char ch : s.toCharArray()) {
        if (ch >= '0' && ch <= '9') {
            //把数字字符转成整型，因为可能不止是个位，而是十位百位，所以需要乘10
            num = num * 10 + (ch - '0');
            continue;
        }
        if (ch == '[') {
            //把数字num压入数字栈中
            numStack.push(num);
            num = 0;
            //把拼接的字符串也压入字符串栈中
            strStack.push(res.toString());
            res = new StringBuilder();
            continue;
        }
        if (ch == ']') {
            StringBuilder temp = new StringBuilder();
            //取出数字栈的数字，进行循环复制
            int count = numStack.pop();
            //拼接
            for (int i = 0; i < count; i++) {
                temp.append(res);
            }
            res = new StringBuilder(strStack.pop() + temp);
            continue;
        }
        //括号[]里的字符
        res.append(ch);
    }
    return res.toString();
}