37 Number of Steps to Reduce a Number to Zero

题目

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

分析

题意：给定一个非负整数，如果它是偶数，那就除以二，如果是奇数，那就减一，直到该数为0；返回操作步数。

题目简单易懂，直接上手。

解答

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

位运算（参考）

class Solution {
    public int numberOfSteps (int num) {
        
        int firstBitMask = 1;
        int steps = 0;
        while(num>0){
            if((num & firstBitMask) >0){
                num &= ~firstBitMask;
            }else{
                num = num >>1; 
            }
            steps ++;
        }
        return steps;
        
    }
}