LCA with access to parent node: done

Author: rampatra

src/main/java/com/ctci/treesandgraphs/BuildOrder.java

@@ -0,0 +1,14 @@
+package com.ctci.treesandgraphs;
+
+/**
+ * @author rampatra
+ * @since 2019-02-21
+ */
+public class BuildOrder {
+
+    
+    
+    public static void main(String[] args) {
+        
+    }
+}

src/main/java/com/ctci/treesandgraphs/LeastCommonAncestorWithParentAccess.java

@@ -0,0 +1,114 @@
+package com.ctci.treesandgraphs;
+
+/**
+ * Design an algorithm and write code to find the first common ancestor of two nodes in a binary
+ * tree. Avoid storing additional nodes in a data structure. Also, for this question, the tree node
+ * has access to its parent node. NOTE: This is not necessarily a binary search tree.
+ *
+ * @author rampatra
+ * @since 2019-02-23
+ */
+public class LeastCommonAncestorWithParentAccess {
+
+    /**
+     * This is a simple approach where we start with two references, one pointing to {@code node a} and another
+     * pointing to {@code node b}. We move the reference pointing to the deeper node upwards, if required, so that
+     * both the references are at the same depth from root. After both the references are at same depth, we simply 
+     * move both the references upwards until they merge. The node at which they merge is our LCA.
+     *
+     * @param a
+     * @param b
+     * @return the least common ancestor node
+     */
+    private static TreeNode findLCA(TreeNode a, TreeNode b) {
+        if (a == null || b == null) {
+            return null;
+        }
+
+        int depthA = depth(a);
+        int depthB = depth(b);
+        // be little careful when both nodes are at same depth
+        TreeNode shallowNode = depthA < depthB ? a : b;
+        TreeNode deeperNode = depthB > depthA ? b : a;
+
+        // move deeper node reference upwards so that both the references are at same depth
+        deeperNode = goUpBy(deeperNode, Math.abs(depthA - depthB));
+
+        while (shallowNode != deeperNode && shallowNode != null && deeperNode != null) {
+            shallowNode = shallowNode.parent;
+            deeperNode = deeperNode.parent;
+        }
+
+        return shallowNode;
+    }
+
+    private static int depth(TreeNode node) {
+        int d = 0;
+        while (node != null && node.parent != null) {
+            d++;
+            node = node.parent;
+        }
+        return d;
+    }
+
+    private static TreeNode goUpBy(TreeNode node, int levelsUp) {
+        int c = 0;
+        while (node != null && c < levelsUp) {
+            node = node.parent;
+            c++;
+        }
+        return node;
+    }
+
+    private static class TreeNode {
+        int val;
+        TreeNode parent;
+        TreeNode left;
+        TreeNode right;
+
+        TreeNode(int val) {
+            this.val = val;
+        }
+    }
+
+    public static void main(String[] args) {
+        /* 
+            The binary tree looks like:
+        
+                     4
+                   /   \
+                  5     8
+                 / \   / \
+                1  3  2  9
+               /       \
+              0        7 
+              
+         */
+        TreeNode treeRoot = new TreeNode(4);
+        treeRoot.left = new TreeNode(5);
+        treeRoot.left.parent = treeRoot;
+        treeRoot.right = new TreeNode(8);
+        treeRoot.right.parent = treeRoot;
+        treeRoot.left.left = new TreeNode(1);
+        treeRoot.left.left.parent = treeRoot.left;
+        treeRoot.left.right = new TreeNode(3);
+        treeRoot.left.right.parent = treeRoot.left;
+        treeRoot.left.left.left = new TreeNode(0);
+        treeRoot.left.left.left.parent = treeRoot.left.left;
+        treeRoot.right.left = new TreeNode(2);
+        treeRoot.right.left.parent = treeRoot.right;
+        treeRoot.right.right = new TreeNode(9);
+        treeRoot.right.right.parent = treeRoot.right;
+        treeRoot.right.left.right = new TreeNode(7);
+        treeRoot.right.left.right.parent = treeRoot.right.left;
+
+        System.out.println("LCA of 0 and 7 is: " + findLCA(treeRoot.left.left.left, treeRoot.right.left.right).val);
+        System.out.println("LCA of 0 and 9 is: " + findLCA(treeRoot.left.left.left, treeRoot.right.right).val);
+        System.out.println("LCA of 0 and 1 is: " + findLCA(treeRoot.left.left.left, treeRoot.left.left).val);
+        System.out.println("LCA of 1 and 2 is: " + findLCA(treeRoot.left.left, treeRoot.right.left).val);
+        System.out.println("LCA of 1 and 7 is: " + findLCA(treeRoot.left.left, treeRoot.right.left.right).val);
+        System.out.println("LCA of 4 and 7 is: " + findLCA(treeRoot, treeRoot.right.left.right).val);
+        System.out.println("LCA of 5 and 2 is: " + findLCA(treeRoot.left, treeRoot.right.left).val);
+        System.out.println("LCA of 7 and 9 is: " + findLCA(treeRoot.right.left.right, treeRoot.right.right).val);
+    }
+}

src/main/java/com/ctci/treesandgraphs/Successor.java

@@ -43,6 +43,17 @@ private static TreeNode getLeftmostNode(TreeNode node) {
         return curr;
     }
 
+    private static class TreeNode {
+        int val;
+        TreeNode parent;
+        TreeNode left;
+        TreeNode right;
+
+        TreeNode(int val) {
+            this.val = val;
+        }
+    }
+
     public static void main(String[] args) {
         /* 
             The binary search tree looks like:
@@ -84,15 +95,4 @@ public static void main(String[] args) {
         System.out.println("InOrder successor of 8 is: " + getInOrderSuccessor(treeRoot.right).val);
         System.out.println("InOrder successor of 9 is: " + getInOrderSuccessor(treeRoot.right.right));
     }
-
-    private static class TreeNode {
-        int val;
-        TreeNode parent;
-        TreeNode left;
-        TreeNode right;
-
-        TreeNode(int val) {
-            this.val = val;
-        }
-    }
 }