Java Chapter 18: Bit Manipulation

java/Bit Manipulation/HammingWeightsOfIntegers.java

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+public class HammingWeightsOfIntegers {
+    public int[] hammingWeightsOfIntegers(int n) {
+        int[] res = new int[n + 1];
+        for (int x = 0; x < n + 1; x++) {
+            res[x] = countSetBits(x);
+        }
+        return res;
+    }
+
+    private int countSetBits(int x) {
+        int count = 0;
+        // Count each set bit of 'x' until 'x' equals 0.
+        while (x > 0) {
+            // Increment the count if the LSB is 1.
+            count += x & 1;
+            // Right shift 'x' to shift the next bit to the LSB position.
+            x >>= 1;
+        }
+        return count;
+    }
+}

java/Bit Manipulation/HammingWeightsOfIntegersDp.java

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+public class HammingWeightsOfIntegersDp {
+    public int[] hammingWeightsOfIntegersDp(int n) {
+        // Base case: the number of set bits in 0 is just 0. We set dp[0] to 
+        // 0 by initializing the entire DP array to 0.
+        int[] dp = new int[n + 1];
+        for (int x = 0; x < n + 1; x++) {
+            // 'dp[x]' is obtained using the result of 'dp[x >> 1]', plus 
+            // the LSB of 'x'.
+            dp[x] = dp[x >> 1] + (x & 1);
+        }
+        return dp;
+    }
+}

java/Bit Manipulation/LonelyInteger.java

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+public class LonelyInteger {
+    public int lonelyInteger(int[] nums) {
+        int res = 0;
+        // XOR each element of the array so that duplicate values will 
+        // cancel each other out (x ^ x == 0).
+        for (int num : nums) {
+            res ^= num;
+        }
+        // 'res' will store the lonely integer because it would not have 
+        // been canceled out by any duplicate.
+        return res;
+    }
+}

java/Bit Manipulation/SwapOddAndEvenBits.java

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+public class SwapOddAndEvenBits {
+    public int swapOddAndEvenBits(int n) {
+        int evenMask = 0x55555555; // 01010101010101010101010101010101
+        int oddMask = 0xAAAAAAAA;  // 10101010101010101010101010101010
+        int evenBits = n & evenMask;
+        int oddBits = n & oddMask;
+        // Shift the even bits to the left, the odd bits to the right, and 
+        // merge these shifted values together.
+        return (evenBits << 1) | (oddBits >> 1);
+    }
+}