# All possible binary numbers of length n with equal sum in both halves

Given a number n, we need to print all n-digit binary numbers with equal sum in left and right halves. If n is odd, then mid element can be either 0 or 1.

Examples:

```Input  : n = 4
Output : 1001 1010 1111

Input : n = 5
Output : 10001 10101 10010 10110 11011 11111
```

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

The idea is to recursively build left and right halves and keep track of difference between counts of 1s in them. We print a string when difference becomes 0 and there are no more characters to add.

## C++

 `// C++ program to generate all binary strings with ` `// equal sums in left and right halves. ` `#include ` `using` `namespace` `std; ` ` `  `/* Default values are used only in initial call. ` `   ``n is number of bits remaining to be filled ` `   ``di is current difference between sums of ` `      ``left and right halves. ` `   ``left and right are current half substrings. */` `void` `equal(``int` `n, string left=``""``, string right=``""``, ` `                                        ``int` `di=0) ` `{ ` `    ``// TWO BASE CASES ` `    ``// If there are no more characters to add (n is 0) ` `    ``if` `(n == 0) ` `    ``{ ` `        ``// If difference between counts of 1s and ` `        ``// 0s is 0 (di is 0) ` `        ``if` `(di == 0) ` `            ``cout << left + right << ``" "``; ` `        ``return``; ` `    ``} ` ` `  `    ``/* If 1 remains than string length was odd */` `    ``if` `(n == 1) ` `    ``{ ` `        ``// If difference is 0, we can put remaining ` `        ``// bit in middle. ` `        ``if` `(di == 0) ` `        ``{ ` `            ``cout << left + ``"0"` `+ right << ``" "``; ` `            ``cout << left + ``"1"` `+ right << ``" "``; ` `        ``} ` `        ``return``; ` `    ``} ` ` `  `    ``/* If difference is more than what can be ` `       ``be covered with remaining n digits ` `       ``(Note that lengths of left and right ` `        ``must be same) */` `    ``if` `((2 * ``abs``(di) <= n)) ` `    ``{ ` ` `  `        ``/*binary number would not start with 0*/` `        ``if` `(left != ``""``) ` `        ``{ ` `            ``/* add 0 to end in both left and right ` `               ``half. Sum in both half will not ` `               ``change*/` `            ``equal(n-2, left+``"0"``, right+``"0"``, di); ` ` `  `            ``/* add 0 to end in both left and right ` `              ``half* subtract 1 from di as right ` `              ``sum is increased by 1*/` `            ``equal(n-2, left+``"0"``, right+``"1"``, di-1); ` `        ``} ` ` `  `        ``/* add 1  in end in left half and 0 to the ` `          ``right half. Add 1 to di as left sum is ` `          ``increased by 1*/` `        ``equal(n-2, left+``"1"``, right+``"0"``, di+1); ` ` `  `        ``/* add 1 in end to both left and right ` `          ``half the sum will not change*/` `        ``equal(n-2, left+``"1"``, right+``"1"``, di); ` `    ``} ` `} ` ` `  `/* driver function */` `int` `main() ` `{ ` `    ``int` `n = 5; ``// n-bits ` `    ``equal(n); ` `    ``return` `0; ` `} `

## Java

 `// Java program to generate all binary strings  ` `// with equal sums in left and right halves. ` `import` `java.util.*; ` ` `  `class` `GFG ` `{ ` ` `  `// Default values are used only in initial call. ` `// n is number of bits remaining to be filled ` `// di is current difference between sums of ` `// left and right halves. ` `// left and right are current half substrings.  ` `static` `void` `equal(``int` `n, String left,  ` `                         ``String right, ``int` `di) ` `{ ` `    ``// TWO BASE CASES ` `    ``// If there are no more characters to add (n is 0) ` `    ``if` `(n == ``0``) ` `    ``{ ` `        ``// If difference between counts of 1s and ` `        ``// 0s is 0 (di is 0) ` `        ``if` `(di == ``0``) ` `            ``System.out.print(left + right + ``" "``); ` `        ``return``; ` `    ``} ` ` `  `    ``/* If 1 remains than string length was odd */` `    ``if` `(n == ``1``) ` `    ``{ ` `        ``// If difference is 0, we can put  ` `        ``// remaining bit in middle. ` `        ``if` `(di == ``0``) ` `        ``{ ` `            ``System.out.print(left + ``"0"` `+ right + ``" "``); ` `            ``System.out.print(left + ``"1"` `+ right + ``" "``); ` `        ``} ` `        ``return``; ` `    ``} ` ` `  `    ``/* If difference is more than what can be ` `    ``be covered with remaining n digits ` `    ``(Note that lengths of left and right ` `     ``must be same) */` `    ``if` `((``2` `* Math.abs(di) <= n)) ` `    ``{ ` ` `  `        ``// binary number would not start with 0 ` `        ``if` `(left != ``""``) ` `        ``{ ` `            ``// add 0 to end in both left and right ` `            ``// half. Sum in both half will not ` `            ``// change ` `            ``equal(n - ``2``, left + ``"0"``, right + ``"0"``, di); ` ` `  `            ``// add 0 to end in both left and right ` `            ``// half* subtract 1 from di as right ` `            ``// sum is increased by 1 ` `            ``equal(n - ``2``, left + ``"0"``, right + ``"1"``, di - ``1``); ` `        ``} ` ` `  `        ``// add 1 in end in left half and 0 to the ` `        ``// right half. Add 1 to di as left sum is ` `        ``// increased by 1* ` `        ``equal(n - ``2``, left + ``"1"``, right + ``"0"``, di + ``1``); ` ` `  `        ``// add 1 in end to both left and right ` `        ``// half the sum will not change ` `        ``equal(n - ``2``, left + ``"1"``, right + ``"1"``, di); ` `    ``} ` `} ` ` `  `// Driver Code ` `public` `static` `void` `main(String args[]) ` `{ ` `    ``int` `n = ``5``;  ` `     `  `    ``// n-bits ` `    ``equal(n, ``""``, ``""``, ``0``); ` `} ` `} ` ` `  `// This code is contributed  ` `// by SURENDRA_GANGWAR `

## Python3

 `# Python program to generate all binary strings with ` `# equal sums in left and right halves. ` ` `  `# Default values are used only in initial call. ` `# n is number of bits remaining to be filled ` `# di is current difference between sums of ` `# left and right halves. ` `# left and right are current half substrings. ` `def` `equal(n: ``int``, left ``=` `"``", right = "``", di ``=` `0``): ` ` `  `    ``# TWO BASE CASES ` `    ``# If there are no more characters to add (n is 0) ` `    ``if` `n ``=``=` `0``: ` ` `  `        ``# If difference between counts of 1s and ` `        ``# 0s is 0 (di is 0) ` `        ``if` `di ``=``=` `0``: ` `            ``print``(left ``+` `right, end ``=` `" "``) ` `        ``return` ` `  `    ``# If 1 remains than string length was odd ` `    ``if` `n ``=``=` `1``: ` ` `  `        ``# If difference is 0, we can put remaining ` `        ``# bit in middle. ` `        ``if` `di ``=``=` `0``: ` `            ``print``(left ``+` `"0"` `+` `right, end ``=` `" "``) ` `            ``print``(left ``+` `"1"` `+` `right, end ``=` `" "``) ` `        ``return` ` `  `    ``# If difference is more than what can be ` `    ``# be covered with remaining n digits ` `    ``# (Note that lengths of left and right ` `    ``# must be same) ` `    ``if` `2` `*` `abs``(di) <``=` `n: ` ` `  `        ``# binary number would not start with 0 ` `        ``if` `left !``=` `"": ` ` `  `            ``# add 0 to end in both left and right ` `            ``# half. Sum in both half will not ` `            ``# change ` `            ``equal(n ``-` `2``, left ``+` `"0"``, right ``+` `"0"``, di) ` ` `  `            ``# add 0 to end in both left and right ` `            ``# half* subtract 1 from di as right ` `            ``# sum is increased by 1 ` `            ``equal(n ``-` `2``, left ``+` `"0"``, right ``+` `"1"``, di ``-` `1``) ` ` `  `        ``# add 1 in end in left half and 0 to the ` `        ``# right half. Add 1 to di as left sum is ` `        ``# increased by 1 ` `        ``equal(n ``-` `2``, left ``+` `"1"``, right ``+` `"0"``, di ``+` `1``) ` ` `  `        ``# add 1 in end to both left and right ` `        ``# half the sum will not change ` `        ``equal(n ``-` `2``, left ``+` `"1"``, right ``+` `"1"``, di) ` ` `  `# Driver Code ` `if` `__name__ ``=``=` `"__main__"``: ` `    ``n ``=` `5` `# n-bits ` `    ``equal(``5``) ` ` `  `# This code is contributed by ` `# sanjeev2552 `

## C#

 `// C# program to generate all binary strings  ` `// with equal sums in left and right halves. ` `using` `System; ` ` `  `class` `GFG ` `{ ` ` `  `// Default values are used only in initial call. ` `// n is number of bits remaining to be filled ` `// di is current difference between sums of ` `// left and right halves. ` `// left and right are current half substrings.  ` `static` `void` `equal(``int` `n, String left,  ` `                         ``String right, ``int` `di) ` `{ ` `    ``// TWO BASE CASES ` `    ``// If there are no more characters ` `    ``// to add (n is 0) ` `    ``if` `(n == 0) ` `    ``{ ` `        ``// If difference between counts of 1s  ` `        ``// and 0s is 0 (di is 0) ` `        ``if` `(di == 0) ` `            ``Console.Write(left + right + ``" "``); ` `        ``return``; ` `    ``} ` ` `  `    ``/* If 1 remains than string length was odd */` `    ``if` `(n == 1) ` `    ``{ ` `        ``// If difference is 0, we can put  ` `        ``// remaining bit in middle. ` `        ``if` `(di == 0) ` `        ``{ ` `            ``Console.Write(left + ``"0"` `+  ` `                          ``right + ``" "``); ` `            ``Console.Write(left + ``"1"` `+  ` `                          ``right + ``" "``); ` `        ``} ` `        ``return``; ` `    ``} ` ` `  `    ``/* If difference is more than what can be ` `    ``be covered with remaining n digits ` `    ``(Note that lengths of left and right ` `    ``must be same) */` `    ``if` `((2 * Math.Abs(di) <= n)) ` `    ``{ ` ` `  `        ``// binary number would not start with 0 ` `        ``if` `(left != ``""``) ` `        ``{ ` `            ``// add 0 to end in both left and right ` `            ``// half. Sum in both half will not ` `            ``// change ` `            ``equal(n - 2, left + ``"0"``, right + ``"0"``, di); ` ` `  `            ``// add 0 to end in both left and right ` `            ``// half* subtract 1 from di as right ` `            ``// sum is increased by 1 ` `            ``equal(n - 2, left + ``"0"``,  ` `                  ``right + ``"1"``, di - 1); ` `        ``} ` ` `  `        ``// add 1 in end in left half and 0 to the ` `        ``// right half. Add 1 to di as left sum is ` `        ``// increased by 1* ` `        ``equal(n - 2, left + ``"1"``,  ` `              ``right + ``"0"``, di + 1); ` ` `  `        ``// add 1 in end to both left and right ` `        ``// half the sum will not change ` `        ``equal(n - 2, left + ``"1"``, right + ``"1"``, di); ` `    ``} ` `} ` ` `  `// Driver Code ` `public` `static` `void` `Main(String []args) ` `{ ` `    ``int` `n = 5;  ` `     `  `    ``// n-bits ` `    ``equal(n, ``""``, ``""``, 0); ` `} ` `} ` ` `  `// This code is contributed by 29AjayKumar `

Output:

```10001 10101 10010 10110 11011 11111
```

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