# Decode a string recursively encoded as count followed by substring

An encoded string (s) is given, the task is to decode it. The pattern in which the strings are encoded is as follows.

```<count>[sub_str] ==> The substring 'sub_str'
appears count times.```

Examples:

```Input : str[] = "1[b]"
Output : b

Input : str[] = "2[ab]"
Output : abab

Input : str[] = "2[a2[b]]"
Output : abbabb

Input : str[] = "3[b2[ca]]"
Output : bcacabcacabcaca
```

## Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

The idea is to use two stacks, one for integers and another for characters.
Now, traverse the string,

1. Whenever we encounter any number, push it into the integer stack and in case of any alphabet (a to z) or open bracket (‘[‘), push it onto the character stack.
2. Whenever any close bracket (‘]’) is encounter pop the character from the character stack until open bracket (‘[‘) is not found in the character stack. Also, pop the top element from the integer stack, say n. Now make a string repeating the popped character n number of time. Now, push all character of the string in the stack. Below is implementation of this approach:

## C++

 `// C++ program to decode a string recursively ` `// encoded as count followed substring ` `#include ` `using` `namespace` `std; ` ` `  `// Returns decoded string for 'str' ` `string decode(string str) ` `{ ` `    ``stack<``int``> integerstack; ` `    ``stack<``char``> stringstack; ` `    ``string temp = ``""``, result = ``""``; ` ` `  `    ``// Traversing the string ` `    ``for` `(``int` `i = 0; i < str.length(); i++) ` `    ``{ ` `        ``int` `count = 0; ` ` `  `        ``// If number, convert it into number ` `        ``// and push it into integerstack. ` `        ``if` `(str[i] >= ``'0'` `&& str[i] <=``'9'``) ` `        ``{ ` `            ``while` `(str[i] >= ``'0'` `&& str[i] <= ``'9'``) ` `            ``{ ` `                ``count = count * 10 + str[i] - ``'0'``; ` `                ``i++; ` `            ``} ` ` `  `            ``i--; ` `            ``integerstack.push(count); ` `        ``} ` ` `  `        ``// If closing bracket ']', pop elemment until ` `        ``// '[' opening bracket is not found in the ` `        ``// character stack. ` `        ``else` `if` `(str[i] == ``']'``) ` `        ``{ ` `            ``temp = ``""``; ` `            ``count = 0; ` ` `  `            ``if` `(! integerstack.empty()) ` `            ``{ ` `                ``count = integerstack.top(); ` `                ``integerstack.pop(); ` `            ``} ` ` `  `            ``while` `(! stringstack.empty() && stringstack.top()!=``'['` `) ` `            ``{ ` `                ``temp = stringstack.top() + temp; ` `                ``stringstack.pop(); ` `            ``} ` ` `  `            ``if` `(! stringstack.empty() && stringstack.top() == ``'['``) ` `                ``stringstack.pop(); ` ` `  `            ``// Repeating the popped string 'temo' count ` `            ``// number of times. ` `            ``for` `(``int` `j = 0; j < count; j++) ` `                ``result = result + temp; ` ` `  `            ``// Push it in the character stack. ` `            ``for` `(``int` `j = 0; j < result.length(); j++) ` `                ``stringstack.push(result[j]); ` ` `  `            ``result = ``""``; ` `        ``} ` ` `  `        ``// If '[' opening bracket, push it into character stack. ` `        ``else` `if` `(str[i] == ``'['``) ` `        ``{ ` `            ``if` `(str[i-1] >= ``'0'` `&& str[i-1] <= ``'9'``) ` `                ``stringstack.push(str[i]); ` ` `  `            ``else` `            ``{ ` `                ``stringstack.push(str[i]); ` `                ``integerstack.push(1); ` `            ``} ` `        ``} ` ` `  `        ``else` `            ``stringstack.push(str[i]); ` `    ``} ` ` `  `    ``// Pop all the elmenet, make a string and return. ` `    ``while` `(! stringstack.empty()) ` `    ``{ ` `        ``result = stringstack.top() + result; ` `        ``stringstack.pop(); ` `    ``} ` ` `  `    ``return` `result; ` `} ` ` `  `// Driven Program ` `int` `main() ` `{ ` `    ``string str = ``"3[b2[ca]]"``; ` `    ``cout << decode(str) << endl; ` `    ``return` `0; ` `} `

## Java

 `// Java program to decode a string recursively ` `// encoded as count followed substring ` ` `  `import` `java.util.Stack; ` ` `  `class` `Test ` `{ ` `    ``// Returns decoded string for 'str' ` `    ``static` `String decode(String str) ` `    ``{ ` `        ``Stack integerstack = ``new` `Stack<>(); ` `        ``Stack stringstack = ``new` `Stack<>(); ` `        ``String temp = ``""``, result = ``""``; ` `      `  `        ``// Traversing the string ` `        ``for` `(``int` `i = ``0``; i < str.length(); i++) ` `        ``{ ` `            ``int` `count = ``0``; ` `      `  `            ``// If number, convert it into number ` `            ``// and push it into integerstack. ` `            ``if` `(Character.isDigit(str.charAt(i))) ` `            ``{ ` `                ``while` `(Character.isDigit(str.charAt(i))) ` `                ``{ ` `                    ``count = count * ``10` `+ str.charAt(i) - ``'0'``; ` `                    ``i++; ` `                ``} ` `      `  `                ``i--; ` `                ``integerstack.push(count); ` `            ``} ` `      `  `            ``// If closing bracket ']', pop elemment until ` `            ``// '[' opening bracket is not found in the ` `            ``// character stack. ` `            ``else` `if` `(str.charAt(i) == ``']'``) ` `            ``{ ` `                ``temp = ``""``; ` `                ``count = ``0``; ` `      `  `                ``if` `(!integerstack.isEmpty()) ` `                ``{ ` `                    ``count = integerstack.peek(); ` `                    ``integerstack.pop(); ` `                ``} ` `      `  `                ``while` `(!stringstack.isEmpty() && stringstack.peek()!=``'['` `) ` `                ``{ ` `                    ``temp = stringstack.peek() + temp; ` `                    ``stringstack.pop(); ` `                ``} ` `      `  `                ``if` `(!stringstack.empty() && stringstack.peek() == ``'['``) ` `                    ``stringstack.pop(); ` `      `  `                ``// Repeating the popped string 'temo' count ` `                ``// number of times. ` `                ``for` `(``int` `j = ``0``; j < count; j++) ` `                    ``result = result + temp; ` `      `  `                ``// Push it in the character stack. ` `                ``for` `(``int` `j = ``0``; j < result.length(); j++) ` `                    ``stringstack.push(result.charAt(j)); ` `      `  `                ``result = ``""``; ` `            ``} ` `      `  `            ``// If '[' opening bracket, push it into character stack. ` `            ``else` `if` `(str.charAt(i) == ``'['``) ` `            ``{ ` `                ``if` `(Character.isDigit(str.charAt(i-``1``))) ` `                    ``stringstack.push(str.charAt(i)); ` `      `  `                ``else` `                ``{ ` `                    ``stringstack.push(str.charAt(i)); ` `                    ``integerstack.push(``1``); ` `                ``} ` `            ``} ` `      `  `            ``else` `                ``stringstack.push(str.charAt(i)); ` `        ``} ` `      `  `        ``// Pop all the elmenet, make a string and return. ` `        ``while` `(!stringstack.isEmpty()) ` `        ``{ ` `            ``result = stringstack.peek() + result; ` `            ``stringstack.pop(); ` `        ``} ` `      `  `        ``return` `result; ` `    ``} ` ` `  `    ``// Driver method ` `    ``public` `static` `void` `main(String args[]) ` `    ``{ ` `        ``String str = ``"3[b2[ca]]"``; ` `        ``System.out.println(decode(str)); ` `    ``} ` `} `

## Python3

 `# Python program to decode a string recursively  ` `# encoded as count followed substring  ` ` `  `# Returns decoded string for 'str'  ` `def` `decode(``Str``): ` `    ``integerstack ``=` `[] ` `    ``stringstack ``=` `[] ` `    ``temp ``=` `"" ` `    ``result ``=` `""  ` ` `  `    ``# Traversing the string  ` `    ``for` `i ``in` `range``(``len``(``Str``)): ` `        ``count ``=` `0` ` `  `        ``# If number, convert it into number  ` `        ``# and push it into integerstack.  ` `        ``if` `(``Str``[i] >``=` `'0'` `and` `Str``[i] <``=``'9'``): ` `            ``while` `(``Str``[i] >``=` `'0'` `and` `Str``[i] <``=` `'9'``): ` `                ``count ``=` `count ``*` `10` `+` `ord``(``Str``[i]) ``-` `ord``(``'0'``)  ` `                ``i ``+``=` `1` ` `  `            ``i ``-``=` `1` `            ``integerstack.append(count) ` ` `  `        ``# If closing bracket ']', pop elemment until  ` `        ``# '[' opening bracket is not found in the  ` `        ``# character stack.  ` `        ``elif` `(``Str``[i] ``=``=` `']'``): ` `            ``temp ``=` `""  ` `            ``count ``=` `0` ` `  `            ``if` `(``len``(integerstack) !``=` `0``): ` `                ``count ``=` `integerstack[``-``1``]  ` `                ``integerstack.pop() ` ` `  `            ``while` `(``len``(stringstack) !``=` `0` `and` `stringstack[``-``1``] !``=``'['` `): ` `                ``temp ``=` `stringstack[``-``1``] ``+` `temp  ` `                ``stringstack.pop() ` ` `  `            ``if` `(``len``(stringstack) !``=` `0` `and` `stringstack[``-``1``] ``=``=` `'['``):  ` `                ``stringstack.pop()  ` ` `  `            ``# Repeating the popped string 'temo' count  ` `            ``# number of times. ` `            ``for` `j ``in` `range``(count): ` `                ``result ``=` `result ``+` `temp  ` ` `  `            ``# Push it in the character stack. ` `            ``for` `j ``in` `range``(``len``(result)): ` `                ``stringstack.append(result[j])  ` ` `  `            ``result ``=` `"" ` ` `  `        ``# If '[' opening bracket, push it into character stack.  ` `        ``elif` `(``Str``[i] ``=``=` `'['``): ` `            ``if` `(``Str``[i``-``1``] >``=` `'0'` `and` `Str``[i``-``1``] <``=` `'9'``):  ` `                ``stringstack.append(``Str``[i])  ` ` `  `            ``else``: ` `                ``stringstack.append(``Str``[i])  ` `                ``integerstack.append(``1``) ` ` `  `        ``else``: ` `            ``stringstack.append(``Str``[i]) ` ` `  `    ``# Pop all the elmenet, make a string and return. ` `    ``while` `len``(stringstack) !``=` `0``: ` `        ``result ``=` `stringstack[``-``1``] ``+` `result  ` `        ``stringstack.pop() ` ` `  `    ``return` `result ` ` `  `# Driven code  ` `if` `__name__ ``=``=` `'__main__'``: ` `    ``Str` `=` `"3[b2[ca]]"` `    ``print``(decode(``Str``)) ` `     `  `# This code is contributed by PranchalK. `

## C#

 `// C# program to decode a string recursively  ` `// encoded as count followed substring  ` `using` `System; ` `using` `System.Collections.Generic; ` ` `  `class` `GFG ` `{ ` `// Returns decoded string for 'str'  ` `public` `static` `string` `decode(``string` `str) ` `{ ` `    ``Stack<``int``> integerstack = ``new` `Stack<``int``>(); ` `    ``Stack<``char``> stringstack = ``new` `Stack<``char``>(); ` `    ``string` `temp = ``""``, result = ``""``; ` ` `  `    ``// Traversing the string  ` `    ``for` `(``int` `i = 0; i < str.Length; i++) ` `    ``{ ` `        ``int` `count = 0; ` ` `  `        ``// If number, convert it into number  ` `        ``// and push it into integerstack.  ` `        ``if` `(``char``.IsDigit(str[i])) ` `        ``{ ` `            ``while` `(``char``.IsDigit(str[i])) ` `            ``{ ` `                ``count = count * 10 + str[i] - ``'0'``; ` `                ``i++; ` `            ``} ` ` `  `            ``i--; ` `            ``integerstack.Push(count); ` `        ``} ` ` `  `        ``// If closing bracket ']', pop elemment  ` `        ``// until '[' opening bracket is not found  ` `        ``// in the character stack.  ` `        ``else` `if` `(str[i] == ``']'``) ` `        ``{ ` `            ``temp = ``""``; ` `            ``count = 0; ` ` `  `            ``if` `(integerstack.Count > 0) ` `            ``{ ` `                ``count = integerstack.Peek(); ` `                ``integerstack.Pop(); ` `            ``} ` ` `  `            ``while` `(stringstack.Count > 0 &&  ` `                   ``stringstack.Peek() != ``'['``) ` `            ``{ ` `                ``temp = stringstack.Peek() + temp; ` `                ``stringstack.Pop(); ` `            ``} ` ` `  `            ``if` `(stringstack.Count > 0 &&  ` `                ``stringstack.Peek() == ``'['``) ` `            ``{ ` `                ``stringstack.Pop(); ` `            ``} ` ` `  `            ``// Repeating the popped string 'temo'  ` `            ``// count number of times.  ` `            ``for` `(``int` `j = 0; j < count; j++) ` `            ``{ ` `                ``result = result + temp; ` `            ``} ` ` `  `            ``// Push it in the character stack.  ` `            ``for` `(``int` `j = 0; j < result.Length; j++) ` `            ``{ ` `                ``stringstack.Push(result[j]); ` `            ``} ` ` `  `            ``result = ``""``; ` `        ``} ` ` `  `        ``// If '[' opening bracket, push it  ` `        ``// into character stack.  ` `        ``else` `if` `(str[i] == ``'['``) ` `        ``{ ` `            ``if` `(``char``.IsDigit(str[i - 1])) ` `            ``{ ` `                ``stringstack.Push(str[i]); ` `            ``} ` ` `  `            ``else` `            ``{ ` `                ``stringstack.Push(str[i]); ` `                ``integerstack.Push(1); ` `            ``} ` `        ``} ` ` `  `        ``else` `        ``{ ` `            ``stringstack.Push(str[i]); ` `        ``} ` `    ``} ` ` `  `    ``// Pop all the elmenet, make a  ` `    ``// string and return.  ` `    ``while` `(stringstack.Count > 0) ` `    ``{ ` `        ``result = stringstack.Peek() + result; ` `        ``stringstack.Pop(); ` `    ``} ` ` `  `    ``return` `result; ` `} ` ` `  `// Driver Code  ` `public` `static` `void` `Main(``string``[] args) ` `{ ` `    ``string` `str = ``"3[b2[ca]]"``; ` `    ``Console.WriteLine(decode(str)); ` `} ` `} ` ` `  `// This code is contributed by Shrikant13 `

Output:

```bcacabcacabcaca
```

<!—-Illustration of above code for “3[b2[ca]]”>
<!—- >

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