[leetcode] 190. Reverse Bits
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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?
这道题是按bit位倒序数字,题目难度为Easy。
比较直观的想法是通过移位操作来获取倒序后的数字,思路比较简单就直接上代码了。具体代码:
class Solution {public: uint32_t reverseBits(uint32_t n) { uint32_t ret = 0; for(int i=0; i<32; ++i) { ret <<= 1; ret |= (n%2); n >>= 1; } return ret; }};
题目追问如果多次调用该如何优化,没想到合适的方法,所以看了下别人的代码,思路比较巧妙,通过位操作来实现上述功能,避免了循环,效率有所提高。具体想法举个栗子大家就清楚了,例如32bit数字0x12345678,通过位操作逐步变化如下:
0x12345678
0x56781234
0x78563412
0x87654321
0x2d951c84
0x1e6a2c48
具体代码:
class Solution {public: uint32_t reverseBits(uint32_t n) { n = ((n & 0xffff0000) >> 16) | ((n & 0x0000ffff) << 16); n = ((n & 0xff00ff00) >> 8) | ((n & 0x00ff00ff) << 8); n = ((n & 0xf0f0f0f0) >> 4) | ((n & 0x0f0f0f0f) << 4); n = ((n & 0xcccccccc) >> 2) | ((n & 0x33333333) << 2); n = ((n & 0xaaaaaaaa) >> 1) | ((n & 0x55555555) << 1); return n; }};
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