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.

Think

• Set a mask:

    Position:   31 30 ..~.. j+1  j ..~.. i  i-1 ..~.. 0
    Bit Val:     1  1  ...   1   0  ...  0   1   ...  1
  

• Use that mask to do & with N, so that in the new N, the position i~j will be zero.

• Left shift i for M to make the position aligned.

• Do | for M and N, then get the final result.

Solution

class Solution {
    /**
     *@param n, m: Two integer
     *@param i, j: Two bit positions
     *return: An integer
     */
    public int updateBits(int n, int m, int i, int j) {

        int mask = 0;
        for(int lfShift = 31; lfShift > j; lfShift--) 
            mask += (1<<lfShift);

        for(int lfShift = i - 1; lfShift >= 0; lfShift--) 
            mask += (1<<lfShift);
        n &= mask;
        m <<= i;
        return n|m;
    }
}