added cses

leetcode/lc1009.cpp

@@ -0,0 +1,27 @@
+class Solution{
+    public : 
+        int bitwiseComplement(int n ){
+            if(n==0) return 1;
+            short c  = bit_width(static_cast<unsigned int>(n));
+            int x = (1<<c) -1;
+            return ~n&x;
+
+        }
+};
+/*
+Idea:
+Flip only the bits used in n's binary representation.
+
+1. Find number of bits in n using:
+   std::bit_width(n)
+   It returns how many bits are needed to represent n.
+
+2. Create a mask with all those bits set to 1:
+   mask = (1 << bit_width) - 1
+   Example: bit_width = 3 -> mask = 111
+
+3. Flip n using ~n, then remove extra leading 1s:
+   result = (~n) & mask
+
+Special case: n = 0 -> answer = 1
+*/