Update Bits

Source

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.

题解

Cracking The Coding Interview 上的题，题意简单来讲就是使用 M 代替 N 中的第i位到第j位。很显然，我们需要借用掩码操作。大致步骤如下：

  1. 得到第i位到第j位的比特位为0，而其他位均为1的掩码mask。
  2. 使用mask与 N 进行按位与，清零 N 的第i位到第j位。
  3. 对 M 右移i位，将 M 放到 N 中指定的位置。
  4. 返回 N | M 按位或的结果。

Java

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 ones = ~0;
        int mask = 0;
        if (j < 31) {
            int left = ones << (j + 1);
            int right = ((1 << i) - 1);
            mask = left | right;
        } else {
            mask = (1 << i) - 1;
        }

        return (n & mask) | (m << i);
    }
}