LeetCode之位操作题java
191. Number of 1 Bits
Write a function that takes an unsigned integer and returns the number of ’1' bits it has (also known as the Hamming weight).
For example, the 32-bit integer ’11' has binary representation 00000000000000000000000000001011
, so the function should return 3.
public class Solution { // you need to treat n as an unsigned value
//使用按位与操作 public int hammingWeight(int n) { int count = 0; while(n!=0){ n = n&(n-1); ++count; } return count; } }
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).
public class Solution { // you need treat n as an unsigned value
//结果往右移,而n值往左移动,以此来匹配 public int reverseBits(int n) { if(n==0) return 0; int result = 0; for(int i=0;i<32;i++){ result <<= 1; if((n&1)==1) result++; n >>= 1; } return result; } }
338. Counting Bits
Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1's in their binary representation and return them as an array.
Example:
For num = 5
you should return [0,1,1,2,1,2]
.
Follow up:
- It is very easy to come up with a solution with run time O(n*sizeof(integer)). But can you do it in linear time O(n) /possibly in a single pass?
- Space complexity should be O(n).
- Can you do it like a boss? Do it without using any builtin function like __builtin_popcount in c++ or in any other language.
使用动态规划的思想;
public class Solution { public int[] countBits(int num) { int[] arr = new int[num+1]; arr[0] = 0; for(int i=1;i<=num;i++){ arr[i] = arr[i&(i-1)]+1;///数i中1的位数,与前面的i&(i-1)中1的位数有关,为i&(i-1)中1的位数+1; } return arr; } }
371. Sum of Two Integers
Calculate the sum of two integers a and b, but you are not allowed to use the operator +
and -
.
Example:
Given a = 1 and b = 2, return 3
不使用加减法对两数进行相加:
解题思路:使用位操作,异或(^)操作(进行不进位的加法),位与(&)操作进行标记待进位的位置,如a=5=0101,b=7=0111,不进位的加法的结果为a^b=0010,待进位的位置是a&b=0101,初次进位0101<<1=1010,与a^b进行相加又产生进位....如此循环,直到进位为0
public class Solution { public int getSum(int a, int b) { int sum = 0; int carry = 0; do{ sum = a^b;//相加不进位 carry = (a&b)<<1;//进位 a = sum; b = carry; }while(b!=0); return a; } }
201. Bitwise AND of Numbers Range
Given a range [m, n] where 0 <= m <= n <= 2147483647, return the bitwise AND of all numbers in this range, inclusive.
For example, given the range [5, 7], you should return 4.
思路:使用n&(n-1),将n最右边的第一个位数为1的经过该操作后变为0,可减少很多运算;
1)当n&(n-1)=0时,直接返回n=0;
2)当n&(n-1)=m时,直接返回m;
3)其他情况,n&(n-1)<m且不为0,最终的结果就是n&(n-1);
public class Solution { public int rangeBitwiseAnd(int m, int n) { while(n>m){ n = n&(n-1); } return n; } }
public class Solution { public int rangeBitwiseAnd(int m, int n) { int step = 0; while(m!=n){ m >>= 1; n >>= 1; step ++; } return m<<step; } }