adding CountingBits

src/Coding Questions/Leetcode/CountingBits.java

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+/**
+ * 338. Counting Bits
+Solved
+Easy
+Topics
+Companies
+Hint
+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.
+
+ 
+
+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 <= 105
+ 
+
+Follow up:
+
+It is very easy to come up with a solution with a runtime of O(n log n). Can you do it in linear time O(n) and possibly in a single pass?
+Can you do it without using any built-in function (i.e., like __builtin_popcount in C++)?
+ */
+
+public class CountingBits {
+    public int[] countBits(int n) {
+        int[] a = new int[n+1];
+        for(int i=1;i<=n;i++){
+            //appraoch one
+            a[i] = a[i >> 1] + (i & 1);
+            //approach two
+            a[i]=Integer.bitCount(i);
+        }
+        return a;
+    }
+}