LC#543, optimal-, diameter of binary tree, O(n) TIME

Author: NirmalSilwal

Leetcode/easy/DiameterOfBinaryTree543.java

@@ -22,6 +22,7 @@ public class TreeNode {
 
 	int ans = 0;
 
+	// approach 1
 	public int diameterOfBinaryTree1(TreeNode root) {
 		if (root == null)
 			return 0;
@@ -52,6 +53,7 @@ private int height(TreeNode node) {
 		return Math.max(lheight, rheight);
 	}
 
+	// approach 2
 	private int height1(TreeNode node) {
 		if (node == null)
 			return -1;
@@ -62,13 +64,13 @@ private int height1(TreeNode node) {
 		return Math.max(lheight, rheight) + 1;
 	}
 
-	public int diameterOfBinaryTree(TreeNode root) {
+	public int diameterOfBinaryTree2(TreeNode root) {
 		if (root == null)
 			return 0;
 		// max distance between two nodes of LHS
-		int leftDia = diameterOfBinaryTree(root.left);
+		int leftDia = diameterOfBinaryTree2(root.left);
 		// max distance between two nodes of RHS
-		int rightDia = diameterOfBinaryTree(root.right);
+		int rightDia = diameterOfBinaryTree2(root.right);
 
 		// diameter that passes through the root node
 		int rootWayDia = height1(root.left) + height1(root.right) + 2;
@@ -78,4 +80,36 @@ public int diameterOfBinaryTree(TreeNode root) {
 		return dia;
 	}
 
+	// 3rd approach
+	static class DiaPair {
+		int pairHeight;
+		int pairDiameter;
+	}
+
+	public int diameterOfBinaryTree(TreeNode root) {
+		DiaPair result = diameter(root);
+		return result.pairDiameter;
+	}
+
+	public DiaPair diameter(TreeNode node) {
+		if (node == null) {
+			DiaPair basePair = new DiaPair();
+			basePair.pairHeight = -1;
+			basePair.pairDiameter = 0;
+			return basePair;
+		}
+
+		DiaPair leftPair = diameter(node.left);
+		DiaPair rightPair = diameter(node.right);
+
+		DiaPair mypair = new DiaPair();
+
+		mypair.pairHeight = Math.max(leftPair.pairHeight, rightPair.pairHeight) + 1;
+
+		// find diameter of mypair
+		int rootWayDia = leftPair.pairHeight + rightPair.pairHeight + 2;
+		mypair.pairDiameter = Math.max(rootWayDia, Math.max(leftPair.pairDiameter, rightPair.pairDiameter));
+
+		return mypair;
+	}
 }