README_EN.md

394. Decode String

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Description

Given an encoded string, return its decoded string.

The encoding rule is: k[encoded_string], where the encoded_string inside the square brackets is being repeated exactly k times. Note that k is guaranteed to be a positive integer.

You may assume that the input string is always valid; No extra white spaces, square brackets are well-formed, etc.

Furthermore, you may assume that the original data does not contain any digits and that digits are only for those repeat numbers, k. For example, there won't be input like 3a or 2[4].

 

Example 1:

Input: s = "3[a]2[bc]"
Output: "aaabcbc"

Example 2:

Input: s = "3[a2[c]]"
Output: "accaccacc"

Example 3:

Input: s = "2[abc]3[cd]ef"
Output: "abcabccdcdcdef"

Example 4:

Input: s = "abc3[cd]xyz"
Output: "abccdcdcdxyz"

 

Constraints:

• 1 <= s.length <= 30
• s consists of lowercase English letters, digits, and square brackets '[]'.
• s is guaranteed to be a valid input.
• All the integers in s are in the range [1, 300].

Solutions

Python3

class Solution:
    def decodeString(self, s: str) -> str:
        s1, s2 = [], []
        num, res = 0, ''
        for c in s:
            if c.isdigit():
                num = num * 10 + int(c)
            elif c == '[':
                s1.append(num)
                s2.append(res)
                num, res = 0, ''
            elif c == ']':
                res = s2.pop() + res * s1.pop()
            else:
                res += c
        return res

Java

class Solution {
    public String decodeString(String s) {
        Deque<Integer> s1 = new ArrayDeque<>();
        Deque<String> s2 = new ArrayDeque<>();
        int num = 0;
        String res = "";
        for (char c : s.toCharArray()) {
            if ('0' <= c && c <= '9') {
                num = num * 10 + c - '0';
            } else if (c == '[') {
                s1.push(num);
                s2.push(res);
                num = 0;
                res = "";
            } else if (c == ']') {
                StringBuilder t = new StringBuilder();
                for (int i = 0, n = s1.pop(); i < n; ++i) {
                    t.append(res);
                }
                res = s2.pop() + t.toString();
            } else {
                res += String.valueOf(c);
            }
        }
        return res;
    }
}

TypeScript

function decodeString(s: string): string {
    let ans = "";
    let stack = [];
    let count = 0; // repeatCount
    for (let cur of s) {
        if (/[0-9]/.test(cur)) {
            count = count * 10 + Number(cur);
        } else if (/[a-z]/.test(cur)) {
            ans += cur;
        } else if ("[" == cur) {
            stack.push([ans, count]);
            // reset
            ans = "";
            count = 0;
        } else {
            // match ']'
            let [pre, count] = stack.pop();
            ans = pre + ans.repeat(count);
        }
    }
    return ans;
}

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