3226. Number of Bit Changes to Make Two Integers Equal

#Problem

You are given two positive integers n and k.

You can choose any bit in the binary representation of n that is equal to 1 and change it to 0.

Return the number of changes needed to make n equal to k. If it is impossible, return -1.

https://leetcode.cn/problems/number-of-bit-changes-to-make-two-integers-equal/

Example 1:

Input: n = 13, k = 4
Output: 2
Explanation:
Initially, the binary representations of n and k are n = 1101₂ and k = 0100₂.
We can change the first and fourth bits of n. The resulting integer is n = 0100₂ = k.

Example 2:

Input: n = 21, k = 21
Output: 0
Explanation:
n and k are already equal, so no changes are needed.

Example 3:

Input: n = 14, k = 13
Output: -1
Explanation:
It is not possible to make n equal to k.

Constraints:

• 1 <= n, k <= 10⁶

#Test Cases

class Solution:
    def minChanges(self, n: int, k: int) -> int:

import pytest

from solution import Solution


@pytest.mark.parametrize('n, k, expected', [
    (13, 4, 2),
    (21, 21, 0),
    (14, 13, -1),
])
class Test:
    def test_solution(self, n, k, expected):
        sol = Solution()
        assert sol.minChanges(n, k) == expected

#Thoughts

先看单个二进制位。当 n = 1, k = 0（$\iff(n\oplus k)\land=1$）时需要修改一次，当 n = 0, k = 1（$\iff(n\oplus k)\land k=1$）时无法成功，其他情况不用修改。

扩展到多个二进制位也是一样的。

所以只要 $(n\oplus k)\land k$ 不为零，就无法成功。否则计算 $(n\oplus k)\land n$ 中 1 的个数，就是需要修改的次数。

计算某个二进制数中 1 的个数，可以按照 191. Number of 1 Bits 中的方法计算。

对于无法成功的情况，只需要 O(1) 时间。对于能成功的情况，设修改次数为 c，则时间复杂度为 O(c)。

#Code

class Solution:
    def minChanges(self, n: int, k: int) -> int:
        if (n ^ k) & k:
            return -1

        r = (n ^ k) & n
        cnt = 0
        while r > 0:
            cnt += 1
            r &= r - 1

        return cnt