# Set the Left most unset bit

• Difficulty Level : Medium
• Last Updated : 28 May, 2022

Given an integer, set the leftmost unset bit. Leftmost unset bit is the first unset bit after most significant set bit. If all bits (after most significant set bit) are set, then return the number.

Examples:

```Input : 10
Output : 14
10 = 1 0 1 0    // 10 binary
14 = 1 1 1 0    // after set left most unset bit

Input : 15
Output : 15
15 = 1 1 1 1    // 15 binary
15 = 1 1 1 1    // because all bits are set```

Approach:-
1. Return the number if all bits are set.
2. Traverse all the bit to get the last unset bit.
3. Take OR with the original number and the unset bit.

Below is the implementation of the approach.

## C++

 `// C++ program to set the leftmost unset bit``#include ``using` `namespace` `std;` `// set left most unset bit``int` `setleftmostunsetbit(``int` `n)``{``    ``// if number contain all``    ``// 1 then return n``    ``if` `((n & (n + 1)) == 0)``        ``return` `n;` `    ``// Find position of leftmost unset bit.``    ``int` `pos = 0;``    ``for` `(``int` `temp=n, count=0; temp>0;``                        ``temp>>=1, count++)` `        ``// if temp L.S.B is zero``        ``// then unset bit pos is``        ``// change``        ``if` `((temp & 1) == 0)``            ``pos = count;  ` `    ``// return OR of number and``    ``// unset bit pos``    ``return` `(n | (1 << (pos)));``}` `// Driver Function``int` `main()``{``    ``int` `n = 10;``    ``cout << setleftmostunsetbit(n);``    ``return` `0;``}`

## Java

 `// Java program to set``// the leftmost unset bit``import` `java.io.*;` `class` `GFG``{``    ``// set left most unset bit``    ``static` `int` `setleftmostunsetbit(``int` `n)``    ``{``        ``// if number contain all``        ``// 1 then return n``        ``if` `((n & (n + ``1``)) == ``0``)``            ``return` `n;``    ` `        ``// Find position of leftmost unset bit.``        ``int` `pos = ``0``;``        ``for` `(``int` `temp = n, count = ``0``; temp > ``0``;``                           ``temp >>= ``1``, count++)``    ` `            ``// if temp L.S.B is zero``            ``// then unset bit pos is``            ``// change``            ``if` `((temp & ``1``) == ``0``)``                ``pos = count;``    ` `        ``// return OR of number and``        ``// unset bit pos``        ``return` `(n | (``1` `<< (pos)));``    ``}``    ` `    ``// Driver Function``    ``public` `static` `void` `main (String[] args)``    ``{``        ``int` `n = ``10``;``        ``System.out.println(setleftmostunsetbit(n));``    ``}``}` `// This code is contributed by Ansu Kumari`

## Python3

 `# Python program to set the leftmost unset bit` `# Set left most unset bit``def` `setleftmostunsetbit(n):``    ``# if number contain all``    ``# 1 then return n``    ``if` `not` `(n & (n ``+` `1``)):``        ``return` `n` `    ``# Find position of leftmost unset bit``    ``pos, temp, count ``=` `0``, n, ``0``    ` `    ``while` `temp:``        ``# if temp L.S.B is zero``        ``# then unset bit pos is``        ``# change``        ``if` `not` `(temp & ``1``):``            ``pos ``=` `count``            ` `        ``count ``+``=` `1``; temp>>``=``1` `    ``# return OR of number and``    ``# unset bit pos``    ``return` `(n | (``1` `<< (pos)))`  `# Driver Function``n ``=` `10``print``(setleftmostunsetbit(n))` `# This code is contributed by Ansu Kumari`

## C#

 `// C# program to set``// the leftmost unset bit``using` `System;` `class` `GFG``{``    ``// set left most unset bit``    ``static` `int` `setleftmostunsetbit(``int` `n)``    ``{``        ``// if number contain all``        ``// 1 then return n``        ``if` `((n & (n + 1)) == 0)``            ``return` `n;``    ` `        ``// Find position of leftmost unset bit.``        ``int` `pos = 0;``        ``for` `(``int` `temp = n, count = 0; temp > 0;``                            ``temp >>= 1, count++)``    ` `            ``// if temp L.S.B is zero``            ``// then unset bit pos is``            ``// change``            ``if` `((temp & 1) == 0)``                ``pos = count;``    ` `        ``// return OR of number and``        ``// unset bit pos``        ``return` `(n | (1 << (pos)));``    ``}``    ` `    ``// Driver Function``    ``public` `static` `void` `Main ()``    ``{``        ``int` `n = 10;``        ``Console.WriteLine(setleftmostunsetbit(n));``    ``}``}` `// This code is contributed by vt_m`

## PHP

 ` 0;``                     ``\$temp` `>>= 1, ``\$count``++)` `        ``// if temp L.S.B is zero``        ``// then unset bit pos is``        ``// change``        ``if` `((``\$temp` `& 1) == 0)``            ``\$pos` `= ``\$count``;` `    ``// return OR of number``    ``// and unset bit pos``    ``return` `(``\$n` `| (1 << (``\$pos``)));``}` `    ``// Driver code``    ``\$n` `= 10;``    ``echo` `setleftmostunsetbit(``\$n``);` `//This code is contributed by mits``?>`

## Javascript

 ``

Output:

`14`

Time Complexity: O(log2n)

Auxiliary Space: O(1)

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