## 394. Decode String

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]`.

## Rust Solution

``````struct Solution;

impl Solution {
fn decode(s: &[char], m: usize, i: &mut usize) -> String {
let mut res = "".to_string();
while *i < m && s[*i] != ']' {
if s[*i].is_digit(10) {
let mut n = 0;
while *i < m && s[*i].is_digit(10) {
n *= 10;
n += (s[*i] as u8 - b'0') as i32;
*i += 1;
}
*i += 1;
let t = Self::decode(s, m, i);
*i += 1;
for _ in 0..n {
res += &t;
}
} else {
res.push(s[*i]);
*i += 1;
}
}
res
}
fn decode_string(s: String) -> String {
let s: Vec<char> = s.chars().collect();
let mut i = 0;
Self::decode(&s, s.len(), &mut i)
}
}

#[test]
fn test() {
let s = "3[a]2[bc]".to_string();
let res = "aaabcbc".to_string();
assert_eq!(Solution::decode_string(s), res);
let s = "3[a2[c]]".to_string();
let res = "accaccacc".to_string();
assert_eq!(Solution::decode_string(s), res);
let s = "2[abc]3[cd]ef".to_string();
let res = "abcabccdcdcdef".to_string();
assert_eq!(Solution::decode_string(s), res);
let s = "abc3[cd]xyz".to_string();
let res = "abccdcdcdxyz".to_string();
assert_eq!(Solution::decode_string(s), res);
}
``````

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