LeetCode75-394-Decode-String

🧠 思路总结

这道题目为什么要用Stack：题目里有嵌套结构（必须先算里面，再算外面），这是LIFO = Stack。

扫描字符串：

1. [：把当前状态压栈
2. ]：弹栈并结算当前层
3. 字母直接拼
4. 数字记录次数

✅ 代码（Java）

class Solution {
    public String decodeString(String s) {
        Stack<Integer> numStack = new Stack<>();
        Stack<String> strStack = new Stack<>();

        String curStr = "";
        int num = 0;

        for (char c : s.toCharArray()) {
            if (Character.isDigit(c)) {
                num = num * 10 + (c - '0');
            } else if (c == '[') {
                numStack.push(num);
                strStack.push(curStr);
                num = 0;
                curStr = "";
            } else if (c == ']') {
                int k = numStack.pop();
                String prev = strStack.pop();
                StringBuilder sb = new StringBuilder(prev);
                for (int i = 0; i < k; i++) {
                    sb.append(curStr);
                }
                curStr = sb.toString();
            } else {
                curStr += c;
            }
        }
        return curStr;
    }
}