Given a range [m, n] where 0 <= m <= n <= 2147483647, return the bitwise AND of all numbers in this range, inclusive.
For example, given the range [5, 7], you should return 4.
int rangeBitwiseAnd(int m, int n) { int mask = 0xffffffff; /* find out the same bits in left side*/ while (mask != 0) { if ((m & mask) == (n & mask)) { break; } mask <<= 1; } return m & mask; }
Idea:
1) we know when a number add one, some of the right bit changes from 0 to 1 or from 1 to 0
2) if a bit is 0, then AND will cause this bit to 0 eventually.
So, we can just simply check how many left bits are same for m and n.
for example:
5 is 101
6 is 110
when 5 adds 1, then the right two bits are changed. the result is 100
6 is 110
7 is 111
when 6 adds 1, then the right one bit is changed. the result is 110.
9 is 1001
10 is 1010
11 is 1011
12 is 1100
Comparing from 9 to 12, we can see the first left bit is same, that's result.