LeetCode 190. Reverse Bits

Posted 2020-07-04

Reverse bits of a given 32 bits unsigned integer.

For example, given input 43261596 (represented in binary as 00000010100101000001111010011100), return 964176192 (represented in binary as00111001011110000010100101000000).

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

Related problem: Reverse Integer

题目：把一个32位的unsigned integer 翻转过来

可以一位一位的移，也可以更多位的移，建立一个hash表即可，我选择的是4位4位的移

 

class Solution {
public:
    uint32_t reverseBits(uint32_t n) {
        //ret用来保存返回值，assist用来辅助
        uint32_t ret = 0,assist=0;
        //建立map，用来保存0~15对应的翻转无符号数
        map<uint32_t, uint32_t> ma;
        ma[0] = 0;ma[1] = 8;ma[2] = 4;ma[3] = 12;ma[4] = 2;ma[5] = 10;ma[6] = 6;ma[7] = 14;
        ma[8] = 1;ma[9] = 9;ma[10] = 5;ma[11] = 13;ma[12] = 3;ma[13] = 11;ma[14] = 7;ma[15] = 15;
        //每次移4位，移7次即可
        for (int i = 0;i < 7;++i)
        {
          //用来得到前4位对应的翻转数
            assist = ma[n & 15ul];
            ret += assist;
            n=n >> 4;
            ret=ret << 4;
        }
        assist = ma[n & 15ul];
        ret += assist;
        return ret;
    }
};