Added palindrome permutation

Author: rampatra

src/main/java/com/ctci/arraysandstrings/PalindromePermutation.java

@@ -0,0 +1,103 @@
+package com.ctci.arraysandstrings;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * @author rampatra
+ * @since 21/11/2018
+ */
+public class PalindromePermutation {
+
+    /**
+     * This method exploits the fact that a palindrome will contain at most
+     * one character with odd counts. All other characters should be of even
+     * counts.
+     *
+     * @param str input string
+     * @return {@code true} if {@code str} is a permutation of a palindrome
+     */
+    private static boolean isPermutationOfPalindrome(String str) {
+        Map<Character, Integer> charCounts = new HashMap<>();
+        Integer freq;
+        int oddCounts = 0; // keep count of odds so that we don't have to loop through the hashmap the second time
+        for (int i = 0; i < str.length(); i++) {
+            char c = str.charAt(i);
+            if (c != 32) {
+                freq = charCounts.get(c) == null ? 0 : charCounts.get(c);
+                if ((freq + 1) % 2 == 1) {
+                    oddCounts++;
+                } else {
+                    oddCounts--;
+                }
+                charCounts.put(c, freq + 1);
+            }
+        }
+        return oddCounts <= 1;
+    }
+
+
+    /**
+     * This approach sets a bit in a number based on the character in the string and
+     * then un-sets the bit if it sees the character again. Finally, checks if the
+     * bitVector has at most one bit set only.
+     *
+     * @param str input string
+     * @return {@code true} if {@code str} is a permutation of a palindrome
+     */
+    private static boolean isPermutationOfPalindromeViaBits(String str) {
+        int bitVector = 0;
+        int index;
+
+        for (int i = 0; i < str.length(); i++) {
+            index = getIndex(str.charAt(i));
+            if (index != -1) {
+                bitVector = toggleBitAt(bitVector, index);
+            }
+        }
+        return (bitVector & (bitVector - 1)) == 0;
+    }
+
+    /**
+     * Calculates the index to set the bit according to the character {@code c}.
+     *
+     * @param c
+     * @return the index to set the bit as per the character {@code c}
+     */
+    private static int getIndex(char c) {
+        char a = 'a';
+        char z = 'z';
+
+        // assuming string contains only lowercase characters
+        if (c < a || c > z) {
+            return -1;
+        }
+
+        return c - a;
+    }
+
+    /**
+     * Toggles the bit at index {@code index} in {@code bitVector}.
+     *
+     * @param bitVector
+     * @param index
+     * @return the resulting {@code bitVector} after toggling the bit
+     */
+    private static int toggleBitAt(int bitVector, int index) {
+        return bitVector ^ (1 << index);
+    }
+
+    public static void main(String[] args) {
+        System.out.println(isPermutationOfPalindrome("tactc oapapa"));
+        System.out.println(isPermutationOfPalindrome("maam"));
+        System.out.println(isPermutationOfPalindrome("maa m"));
+        System.out.println(isPermutationOfPalindrome("rammmar"));
+        System.out.println(isPermutationOfPalindrome("rammmara"));
+        System.out.println("---------");
+        System.out.println(isPermutationOfPalindromeViaBits("tactc oapapa"));
+        System.out.println(isPermutationOfPalindromeViaBits("maam"));
+        System.out.println(isPermutationOfPalindromeViaBits("maa m"));
+        System.out.println(isPermutationOfPalindromeViaBits("rammmar"));
+        System.out.println(isPermutationOfPalindromeViaBits("rammmara"));
+    }
+}