LeetCode 190 Reverse Bits

题目描述

Reverse bits of a given 32 bits unsigned integer.

For example, given input 43261596 (represented in binary as 00000010100101000001111010011100), return 964176192 (represented in binary as 00111001011110000010100101000000).

Follow up:
If this function is called many times, how would you optimize it?

Related problem: Reverse Integer

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

分析

JDK ：int reverse(int i)

    /**     * Returns the value obtained by reversing the order of the bits in the     * two's complement binary representation of the specified {@code int}     * value.     *     * @param i the value to be reversed     * @return the value obtained by reversing order of the bits in the     *     specified {@code int} value.     * @since 1.5     */    public static int reverse(int i) {        // HD, Figure 7-1        i = (i & 0x55555555) << 1 | (i >>> 1) & 0x55555555;        i = (i & 0x33333333) << 2 | (i >>> 2) & 0x33333333;        i = (i & 0x0f0f0f0f) << 4 | (i >>> 4) & 0x0f0f0f0f;        i = (i << 24) | ((i & 0xff00) << 8) |            ((i >>> 8) & 0xff00) | (i >>> 24);        return i;    }

思路

把第i位取出，放到结果的第31-i位上。

代码

    public static int reverseBits(int n) {        int rt = 0;        for (int i = 0; i < 32; i++) {            rt |= ((n >> i) & 1) << (31 - i);        }        return rt;    }