{leetcode}/problems/counting-bits/[LeetCode - 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.
Input: 2 Output: [0,1,1]
Input: 5 Output: [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.
从 0 开始，每个奇数包含 1 的个数，都比前面一个数要多 1。所以，只需要计算偶数包含的 1 的个数，然后奇数直接在前面的基础上加 1 即可。
另外一种解法：以 2 的 x 次幂为界，后面的的 x 个数中的 1 的个数，是前面个 x 个数在前面加 1 的结果（位数不够在前面补零，例如从 1 到 5，就是从 1 补零 001，再前面加 1，则为 1001）。
\begin{aligned} (0)&=(0)_{2}\\ (1)&=(1)_{2}\\ (2)&=(10)_{2}\\ (3)&=(11)_{2}\\ P(x+b)&=P(x)+1, b=2^{m}>x\\ \end{aligned}
for (int i = 0; i < 99; i++) {
int j = 0;
for (; Math.pow(2, j) <= i; j++) {
if (Math.pow(2, j) == i && i != 1) {
System.out.println("---");
}
}
System.out.printf("%2d %10s\n", i, Integer.toBinaryString(i));
}用这段代码输出一下，观察一下更明显。
这个题的解法跟 89. Gray Code 有异曲同工之妙！
思考题：尝试"动态规划 + 最低有效位"的解法。
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 1:
Input: 2 Output: [0,1,1]
Example 2:
Input: 5
Output: [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.
link:{sourcedir}/_0338_CountingBits.java[role=include]