Longest set bits done

Author: rampatra

src/main/java/com/ctci/bitmanipulation/BinaryToString.java

@@ -28,7 +28,9 @@ private static String decimalFractionToBinaryString(double realNum) {
                 return "ERROR";
             }
             realNum = realNum * 2;
+            // the binary bit is the whole number part (left to the decimal)
             binaryBit = (int) realNum;
+            // we only have to take the part after the decimal (right to the decimal) for the next iteration
             if (binaryBit == 1) {
                 realNum -= 1;
             }

src/main/java/com/ctci/bitmanipulation/FlipBitToWin.java

@@ -0,0 +1,52 @@
+package com.ctci.bitmanipulation;
+
+/**
+ * @author rampatra
+ * @since 2019-03-16
+ */
+public class FlipBitToWin {
+
+    /**
+     * You have an integer and you can flip exactly one bit from a O to a 1. Write code to find the length of the
+     * longest sequence of 1s you could create.
+     * Example:
+     * Input: 1775 (or: 11011101111) Output: 8
+     * <p>
+     * Approach:
+     * We just walk through the integer tracking the current 1s sequence length and the previous 1s sequence length.
+     * When we see a zero, update previous length as follows:
+     * - If the next bit is a 1, previous Length should be set to current Length.
+     * - If the next bit is a 0, then we can't merge these sequences together. So, set previous Length to 0.
+     *
+     * @param n an integer
+     * @return the longest sequence of set bits in {@code n} by flipping only one zero bit
+     */
+    private static int findLongestSequence(int n) {
+        // if all bits are set, return the total number of bits in an integer
+        if (n == ~0) {
+            return Integer.BYTES * 8;
+        }
+
+        int prevOnesLen = 0;
+        int currOnesLen = 0;
+        int maxOnesLen = 0;
+
+        while (n > 0) {
+            // if the current bit is 0, reset the currOnesLen
+            if ((n & 1) == 0) {
+                prevOnesLen = (n & 2) == 0 ? 0 : currOnesLen; // if the next bit is also 0, set prevOnesLen to 0
+                currOnesLen = 0;
+            } else {
+                currOnesLen++;
+            }
+            n >>>= 1;
+            maxOnesLen = Math.max(maxOnesLen, prevOnesLen + 1 + currOnesLen);
+        }
+        return maxOnesLen;
+    }
+
+    public static void main(String[] args) {
+        System.out.println("Longest seq in " + Integer.toBinaryString(125) + " is " + findLongestSequence(125));
+        System.out.println("Longest seq in " + Integer.toBinaryString(1275) + " is " + findLongestSequence(1275));
+    }
+}