bitwise-and-number-range

Problem

Bitwise AND of Numbers Range

Problem Description

Given a range [m, n] where 0 <= m <= n <= 2147483647, return the bitwise AND of all numbers in this range, inclusive.

Example 1:

Input: [5,7]
Output: 4
Example 2:

Input: [0,1]
Output: 0

Solution

This is a Bit operation problem, and I feel helpless every time when I see bit operation... anyway, look at this problem, we can find out that it needed to bit AND to all the number for given range, for example [4,5], it asking we do 4 (100) AND 5 (101) = 100 4(100), bit AND, only 1 & 1 = 1, otherwise is 0. for we can see we need to find out the highest common 1s(left most common 1s) in the range.

i.e. [4,6] -- the left most common 1s is 100 = 4.

 4     5    6
100   101  110

How to find the highest common 1s.

• if m != n, left shift m and n, m>>=1, n>>=1

• count shifts for each shift

• until m==n, found left most common 1s,

• the result is left most common 1s right shift shiftCounts.

Code

class Solution {
    public int rangeBitwiseAnd(int m, int n) {
        if (m == 0) return 0;
        int shift = 0;
        while (m != n) {
            m >>= 1;
            n >>= 1;
            shift++;
        }
        return m << shift;
    }
}