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Formatted question description: https://leetcode.ca/all/2220.html

# 2220. Minimum Bit Flips to Convert Number (Easy)

A bit flip of a number x is choosing a bit in the binary representation of x and flipping it from either 0 to 1 or 1 to 0.

• For example, for x = 7, the binary representation is 111 and we may choose any bit (including any leading zeros not shown) and flip it. We can flip the first bit from the right to get 110, flip the second bit from the right to get 101, flip the fifth bit from the right (a leading zero) to get 10111, etc.

Given two integers start and goal, return the minimum number of bit flips to convert start to goal.

Example 1:

Input: start = 10, goal = 7
Output: 3
Explanation: The binary representation of 10 and 7 are 1010 and 0111 respectively. We can convert 10 to 7 in 3 steps:
- Flip the first bit from the right: 1010 -> 1011.
- Flip the third bit from the right: 1011 -> 1111.
- Flip the fourth bit from the right: 1111 -> 0111.
It can be shown we cannot convert 10 to 7 in less than 3 steps. Hence, we return 3.

Example 2:

Input: start = 3, goal = 4
Output: 3
Explanation: The binary representation of 3 and 4 are 011 and 100 respectively. We can convert 3 to 4 in 3 steps:
- Flip the first bit from the right: 011 -> 010.
- Flip the second bit from the right: 010 -> 000.
- Flip the third bit from the right: 000 -> 100.
It can be shown we cannot convert 3 to 4 in less than 3 steps. Hence, we return 3.


Constraints:

• 0 <= start, goal <= 109

Similar Questions:

## Solution 1.

XOR sets the bits that are different between start and goal, and unsets bits that are the same.

__builtin_popcount(mask) counts the 1s in mask.

Example:

## Expression | Value

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|– start| 0011010 goal| 0101100 start^goal| 0110110 __builtin_popcount(start^goal) | 4

• // OJ: https://leetcode.com/problems/minimum-bit-flips-to-convert-number/
// Time: O(1)
// Space: O(1)
class Solution {
public:
int minBitFlips(int start, int goal) {
return __builtin_popcount(start ^ goal);
}
};

• class Solution:
def minBitFlips(self, start: int, goal: int) -> int:
t = start ^ goal
ans = 0
while t:
ans += t & 1
t >>= 1
return ans

############

# 2220. Minimum Bit Flips to Convert Number
# https://leetcode.com/problems/minimum-bit-flips-to-convert-number/

class Solution:
def minBitFlips(self, start: int, goal: int) -> int:
a = start ^ goal
count = 0

while a:
count += a & 1
a >>= 1

return count


• class Solution {
public int minBitFlips(int start, int goal) {
int t = start ^ goal;
int ans = 0;
while (t != 0) {
ans += t & 1;
t >>= 1;
}
return ans;
}
}

• func minBitFlips(start int, goal int) int {
t := start ^ goal
ans := 0
for t != 0 {
ans += t & 1
t >>= 1
}
return ans
}

• function minBitFlips(start: number, goal: number): number {
let tmp = start ^ goal;
let ans = 0;
while (tmp !== 0) {
ans += tmp & 1;
tmp >>= 1;
}
return ans;
}


• impl Solution {
pub fn min_bit_flips(start: i32, goal: i32) -> i32 {
let mut tmp = start ^ goal;
let mut ans = 0;
while tmp != 0 {
ans += tmp & 1;
tmp >>= 1;
}
ans
}
}



## Discuss

https://leetcode.com/problems/minimum-bit-flips-to-convert-number/discuss/1907384/