栈系列一＞字符串解码

class Solution {
    public String decodeString(String _s) {
        char[] s = _s.toCharArray();
        int n = s.length;
        int i = 0;
        Stack<Integer> nums = new Stack<>();
        Stack<StringBuffer> st = new Stack<>();
        //先放一个空串进去，防止越界访问
        st.push(new StringBuffer());

        while(i < n){
            if(s[i] >= '0' && s[i] <= '9'){
                int tmp = 0;
                while(i < n && Character.isDigit(s[i])){
                    tmp = tmp * 10 + (s[i]-'0');
                    i++;
                }
                nums.push(tmp);
            }

            else if(s[i] == '['){
                //把'['后面的字符串提取出来，放入st
                i++;
                StringBuffer tmp = new StringBuffer();
                while(i < n && s[i] >= 'a' && s[i] <= 'z'){
                    tmp.append(s[i]);
                    i++;
                }
                st.push(tmp);
            }

            else if(s[i] == ']'){
                //解析
                int k = nums.pop();
                StringBuffer tmp = st.pop();
                while(k > 0){
                    st.peek().append(tmp);
                    k--;
                }
                i++;
            }else{
                //单独遇到字符串,提取字符放入st栈顶字符串后面
                StringBuffer tmp = new StringBuffer();
                while(i < n && s[i] >= 'a' && s[i] <= 'z'){
                    tmp.append(s[i]);
                    i++;
                }
                st.peek().append(tmp);
            }
        }

        return st.peek().toString();
    }
}