3064 - Guess the Number Using Bitwise Questions I (Medium)

Problem Link

https://leetcode.com/problems/guess-the-number-using-bitwise-questions-i/

Problem Statement

There is a number n that you have to find.

There is also a pre-defined API int commonSetBits(int num), which returns the number of bits where both n and num are 1 in that position of their binary representation. In other words, it returns the number of set bits in n & num, where & is the bitwise AND operator.

Return the number n.

Example 1:

Input: n = 31

Output: 31

Explanation: It can be proven that it's possible to find 31 using the provided API.

Example 2:

Input: n = 33

Output: 33

Explanation: It can be proven that it's possible to find 33 using the provided API.

Constraints:

• $1 <= n <= 2 ^ 30 - 1$
• $0 <= num <= 2 ^ 30 - 1$
• If you ask for some num out of the given range, the output wouldn't be reliable.

Approach 1: Bit Manipulation

/**
 * Definition of commonSetBits API.
 * int commonSetBits(int num);
 */

class Solution {
public:
    int findNumber() {
        int ans = 0;
        // ask for 2 ^ i for 0 <= i < 30
        for (int i = 0; i < 30; i++) {
            // since we only have one bit set in `num`,
            // commonSetBits returns either 0 or 1
            // e.g. 1010 & 0001 -> 0
            // e.g. 1011 & 0001 -> 1
            if (commonSetBits(1 << i)) {
                // if it is 1, then that means this bit in `n` is set
                // then use OR operator to set the bit to `ans`
                ans |= (1 << i);
            }
        }
        return ans;
    }
};