236. Decode String

[...] segments can nest. When we hit [, we freeze the current state (the string built so far and the multiplier) on a stack, and start a fresh inner build. When we hit ], we pop and splice the inner k times into the parent.

Approach

Maintain curr_str and curr_k (the number just being parsed).

import java.util.*;

class Solution {
    public String decodeString(String s) {
        Deque<String> strStack = new ArrayDeque<>();
        Deque<Integer> countStack = new ArrayDeque<>();
        StringBuilder curr = new StringBuilder();
        int k = 0;
        for (char c : s.toCharArray()) {
            if (Character.isDigit(c)) {
                k = k * 10 + (c - '0');
            } else if (c == '[') {
                strStack.push(curr.toString());
                countStack.push(k);
                curr = new StringBuilder();
                k = 0;
            } else if (c == ']') {
                String prev = strStack.pop();
                int repeat = countStack.pop();
                StringBuilder tmp = new StringBuilder(prev);
                for (int i = 0; i < repeat; i++) {
                    tmp.append(curr);
                }
                curr = tmp;
            } else {
                curr.append(c);
            }
        }
        return curr.toString();
    }
}