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dsa-solutions/lc-solutions/0300-0399/338-counting-bits.md

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+---
+id: counting-bits
+title: Counting Bits
+sidebar_label: 338-Counting Bits
+tags:
+  - Arrays
+  - Bit Manipulation
+  - LeetCode
+  - Java
+  - Python
+  - C++
+description: "This is a solution to the Counting Bits problem on LeetCode."
+sidebar_position: 5
+---
+
+## Problem Description
+
+Given an integer `n`, return an array `ans` of length `n + 1` such that for each `i` (0 <= `i` <= `n`), `ans[i]` is the number of `1`s in the binary representation of `i`.
+
+### Examples
+
+**Example 1:**
+
+```
+Input: n = 2
+Output: [0, 1, 1]
+Explanation:
+0 --> 0
+1 --> 1
+2 --> 10
+```
+
+**Example 2:**
+
+```
+Input: n = 5
+Output: [0, 1, 1, 2, 1, 2]
+Explanation:
+0 --> 0
+1 --> 1
+2 --> 10
+3 --> 11
+4 --> 100
+5 --> 101
+```
+
+### Constraints
+
+- `0 <= n <= 10^5`
+
+### Follow-up
+
+- Can you solve it in linear time `O(n)` and possibly in a single pass?
+- Can you solve it without using any built-in function (i.e., like `__builtin_popcount` in C++)?
+
+---
+
+## Solution for Counting Bits Problem
+
+### Approach 1: Brute Force (Naive)
+
+The brute force approach involves iterating through each number from `0` to `n`, converting each number to its binary form, and counting the number of `1`s in that binary representation.
+
+#### Code in Different Languages
+
+<Tabs>
+<TabItem value="C++" label="C++" default>
+<SolutionAuthor name="@ImmidiSivani"/>
+
+```cpp
+class Solution {
+public:
+    vector<int> countBits(int n) {
+        vector<int> ans(n + 1);
+        for (int i = 0; i <= n; ++i) {
+            ans[i] = __builtin_popcount(i);
+        }
+        return ans;
+    }
+};
+```
+
+</TabItem>
+<TabItem value="Java" label="Java">
+<SolutionAuthor name="@ImmidiSivani"/>
+
+```java
+class Solution {
+    public int[] countBits(int n) {
+        int[] ans = new int[n + 1];
+        for (int i = 0; i <= n; ++i) {
+            ans[i] = Integer.bitCount(i);
+        }
+        return ans;
+    }
+}
+```
+
+</TabItem>
+<TabItem value="Python" label="Python">
+<SolutionAuthor name="@ImmidiSivani"/>
+
+```python
+class Solution:
+    def countBits(self, n: int) -> List[int]:
+        return [bin(i).count('1') for i in range(n + 1)]
+```
+
+</TabItem>
+</Tabs>
+
+#### Complexity Analysis
+
+- **Time Complexity**: $O(nlogn)$, as converting a number to binary and counting bits takes $O(\log n)$ time, and we do this for each number from `0` to `n`.
+- **Space Complexity**: $O(n)$, for storing the result array.
+
+### Approach 2: Optimized Dynamic Programming
+
+To achieve a linear time solution, we can use a dynamic programming approach. We use the property that the number of `1`s in `i` can be derived from the number of `1`s in `i >> 1` (i.e., `i` divided by `2`) plus `1` if the last bit of `i` is `1`.
+
+#### Code in Different Languages
+
+<Tabs>
+<TabItem value="C++" label="C++" default>
+<SolutionAuthor name="@ImmidiSivani"/>
+
+```cpp
+class Solution {
+public:
+    vector<int> countBits(int n) {
+        vector<int> ans(n + 1);
+        for (int i = 1; i <= n; ++i) {
+            ans[i] = ans[i >> 1] + (i & 1);
+        }
+        return ans;
+    }
+};
+```
+
+</TabItem>
+<TabItem value="Java" label="Java">
+<SolutionAuthor name="@ImmidiSivani"/>
+
+```java
+class Solution {
+    public int[] countBits(int n) {
+        int[] ans = new int[n + 1];
+        for (int i = 1; i <= n; ++i) {
+            ans[i] = ans[i >> 1] + (i & 1);
+        }
+        return ans;
+    }
+}
+```
+
+</TabItem>
+<TabItem value="Python" label="Python">
+<SolutionAuthor name="@ImmidiSivani"/>
+
+```python
+class Solution:
+    def countBits(self, n: int) -> List[int]:
+        ans = [0] * (n + 1)
+        for i in range(1, n + 1):
+            ans[i] = ans[i >> 1] + (i & 1)
+        return ans
+```
+
+</TabItem>
+</Tabs>
+
+#### Complexity Analysis
+
+- **Time Complexity**: $O(n)$, as we calculate the number of bits for each number from `0` to `n` in constant time.
+- **Space Complexity**: $O(n)$, for storing the result array.
+
+---
+
+<h2>Authors:</h2>
+
+<div style={{display: 'flex', flexWrap: 'wrap', justifyContent: 'space-between', gap: '10px'}}>
+{['ImmidiSivani'].map(username => (
+ <Author key={username} username={username} />
+))}
+</div>