Update Bits

Given two 32-bit numbers, N and M, and two bit positions, i and j. Write a method to set all bits between i and j in N equal to M (e g , M becomes a substring of N located at i and starting at j)

Example

Given N=(10000000000)2, M=(10101)2, i=2, j=6

return N=(10001010100)2

Note

In the function, the numbers N and M will given in decimal, you should also return a decimal number.

Challenge

Minimum number of operations?

Clarification

You can assume that the bits j through i have enough space to fit all of M. That is, if M=10011， you can assume that there are at least 5 bits between j and i. You would not, for example, have j=3 and i=2, because M could not fully fit between bit 3 and bit 2.

Solution

• Time O(1)

Buid a Mask: 11111100000011111 in [i,j], the bits are all 0

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class Solution:
    #@param n, m: Two integer
    #@param i, j: Two bit positions
    #return: An integer
    def updateBits(self, n, m, i, j):
        # write your code here
        # Build the mask
        if j < 31:
            ones = ~0 # 2**32 - 1
            # [0,end] = 11111111100000000000000
            left = ones << (j+1) # Note, you shit j+1 bit to make [i,j] is 0
            # [0,i) = 111111
            right = (1 << i) - 1
            # mask = 11111111100000000111111
            mask = left | right
        else: # j+1 = 32 (> max_int), then mask = right
            mask = (1 << i) - 1
        return (mask & n) | (m<<i)