add: LowestCommonAncestor

Author: Jer3myYu

java/Trees/LowestCommonAncestor.java

@@ -0,0 +1,41 @@
+import DS.TreeNode;
+
+/*
+    // Definition of TreeNode:
+    class TreeNode {
+        public int val;
+        public TreeNode left;
+        public TreeNode right;
+        public TreeNode(int val) {
+            this.val = val;
+        }
+    }
+ */
+
+public class LowestCommonAncestor {
+    TreeNode lca;
+
+    public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
+        dfs(root, p, q);
+        return lca;
+    }
+
+    private boolean dfs(TreeNode node, TreeNode p, TreeNode q) {
+        // Base case: a null node is neither 'p' nor 'q'.
+        if (node == null) {
+            return false;
+        }
+        int nodeIsPOrQ = (node == p || node == q) ? 1 : 0;
+        // Recursively determine if the left and right subtrees contain 'p' 
+        // or 'q'.
+        int leftContainsPOrQ = dfs(node.left, p, q) ? 1 : 0;
+        int rightContainsPOrQ = dfs(node.right, p, q) ? 1 : 0;
+        // If two of the above three variables are true, the current node is 
+        // the LCA.
+        if (nodeIsPOrQ + leftContainsPOrQ + rightContainsPOrQ == 2) {
+            lca = node;
+        }
+        // Return true if the current subtree contains 'p' or 'q'.
+        return nodeIsPOrQ + leftContainsPOrQ + rightContainsPOrQ > 0;
+    }
+}