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_190.java
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package com.fishercoder.solutions;
/**190. Reverse Bits
*
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?*/
public class _190 {
/**This post: http://stackoverflow.com/questions/2811319/difference-between-and
* gives a good explanation between logical right shift: ">>>" and arithmetic right shift: ">>".
* Basically, ">>" preserves the most left bit and treats it as the sign for this number,
* e.g. -2 represented in 8 bits is 11111110, thus -2 >> 1 will become 11111111, i.e. -1
* notice its sign bit (the most left one bit) is preserved
* However, logical right shift ">>>" doesn't care about the first bit on the most left,
* it simply shifts every bit to the right.
* e.g. -2 >>> 1 would become 1111111111111111111111111111111, i.e. 2147483647*/
// you need treat n as an unsigned value
public int reverseBits(int n) {
int res = 0;
for(int i = 0; i < 32; i++){
res += n&1;//get the most right bit each time
n = n >>> 1;//do UN-signed right shift by 1 each time
if(i < 31) res = res << 1;//shift this number to the left by 1 each time, so that eventually, this number is reversed
}
return res;
}
//follow-up: if this function is called many times, how to improve it?
//Divide the integer into 4 bytes, reverse each byte and then combine them into one in the end, use cache to store the reversed results for reuse if possible.
public static void main(String...strings){
System.out.println(Integer.toBinaryString(4));
_190 test = new _190();
int n = 1;
System.out.println(test.reverseBits(n));
System.out.println(Integer.parseInt("11000", 2));
System.out.println(Integer.parseInt("00011", 2));
// System.out.println(-2 >>> 1);
// System.out.println(Integer.toBinaryString(-2 >>> 1));
// System.out.println(Integer.toBinaryString(-2));
System.out.println(Integer.toBinaryString(-1));
System.out.println(Integer.toBinaryString(6));
}
}