Given a non-negative integer num, return the number of steps to reduce it to zero. If the current number is even, you have to divide it by 2, otherwise, you have to subtract 1 from it.

Example 1:
Input: num = 14
Output: 6

Explanation:
Step 1) 14 is even; divide by 2 and obtain 7.
Step 2) 7 is odd; subtract 1 and obtain 6.
Step 3) 6 is even; divide by 2 and obtain 3.
Step 4) 3 is odd; subtract 1 and obtain 2.
Step 5) 2 is even; divide by 2 and obtain 1.
Step 6) 1 is odd; subtract 1 and obtain 0.
Example 2:

Input: num = 8
Output: 4
Explanation:
Step 1) 8 is even; divide by 2 and obtain 4.
Step 2) 4 is even; divide by 2 and obtain 2.
Step 3) 2 is even; divide by 2 and obtain 1.
Step 4) 1 is odd; subtract 1 and obtain 0.
Example 3:

Input: num = 123
Output: 12

Constraints:
0 <= num <= 10^6

Hints:
Simulate the process to get the final answer.

Iterative Approach

We can simulate the process until we make the number zero (simple, intuitive and effective).

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class Solution {
public:
    int numberOfSteps (int num) {
        int r = 0;
        while (num != 0) {
            r ++;
            if (num % 2 == 0) {
                num >>= 1;
            } else {
                num --;
            }
        }
        return r;
    }
};

class Solution {
public:
    int numberOfSteps (int num) {
        int r = 0;
        while (num != 0) {
            r ++;
            if (num % 2 == 0) {
                num >>= 1;
            } else {
                num --;
            }
        }
        return r;
    }
};

Recursive Algorithm

Alternatively, we can do this recursively but this approach is at the risk of stack-overflow.

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class Solution {
public:
    int numberOfSteps (int num) {
        return num == 0 ? 0 : 1 + 
                numberOfSteps(num % 2 == 0 ? num / 2 : num - 1);
    }
};

class Solution {
public:
    int numberOfSteps (int num) {
        return num == 0 ? 0 : 1 + 
                numberOfSteps(num % 2 == 0 ? num / 2 : num - 1);
    }
};

–EOF (The Ultimate Computing & Technology Blog) —