Expression Expand

Given an expression s includes numbers, letters and brackets. Number represents the number of repetitions inside the brackets(can be a string or another expression)．Please expand expression to be a string.

Have you met this question in a real interview? 

Yes

Example

s = abc3[a] return abcaaa
s = 3[abc] return abcabcabc
s = 4[ac]dy, return acacacacdy
一個必須要是用stack來處理的問題

java

public class Solution {
    /*
     * @param s: an expression includes numbers, letters and brackets
     * @return: a string
     */
    public String expressionExpand(String s) {
        // write your code here
        if (s == null || s.length() == 0) {
            return "";
        }
        Stack<Object> stack = new Stack<>();
        int num = 0;
        for (int i = 0; i < s.length(); i++) {
            if (Character.isLetter(s.charAt(i))) {
                stack.push(s.charAt(i) + "");
            } else if (Character.isDigit(s.charAt(i))) {
                num = num * 10 + s.charAt(i) - '0';
            } else if (s.charAt(i) == '[') {
                stack.push(num);
                num = 0;
            } else if (s.charAt(i) == ']') {
                transfer(stack);
            }
        }
        StringBuffer sb = new StringBuffer();
        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }
        return sb.reverse().toString();
    }
    private void transfer(Stack<Object> stack) {
        StringBuffer sb = new StringBuffer();
        int num = 0;
        while (!stack.isEmpty()) {
            Object obj = stack.pop();
            if (obj instanceof String) {
                sb.append((String)obj);
            } else {
                num = (int)obj;
                break;
            }
        }
        sb = sb.reverse();
        for (int i = 0; i < num; i++) {
            for (int j = 0; j < sb.length(); j++) {
                stack.push(sb.charAt(j) + "");
            }
        }
    }
}