Maximum Depth of Binary Tree

MaximumDepthOfBinaryTree.java

/**
 * Given a binary tree, find its maximum depth.
 * The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.
 */
 
/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int maxDepthRecursive(TreeNode root) {
        if(root == null) return 0;
        int left_Depth = maxDepthRecursive(root.left);
        int right_Depth = maxDepthRecursive(root.right);
        return (left_Depth > right_Depth) ? left_Depth + 1 : right_Depth + 1;
    }
    
    
}

MaximumDepthOfBinaryTreeIter.java

/**
 * Postorder iterative traversal 
 */
 
public class Solution{
    public int maxDepthIterative(TreeNode root){
        if(root == null) return 0;
        Stack<TreeNode> s = new Stack<TreeNode>();
        s.push(root);
        int maxDepth = 0;
        TreeNode prev = null;
        while(!s.isEmpty()){
            TreeNode curr = s.peek();
            if(prev == null || prev.left == curr || prev.right == curr){
                if(curr.left != null) s.push(curr.left);
                else if(curr.right != null) s.push(curr.right);
            }else if(curr.left == prev){
                if(curr.right != null) s.push(curr.right);
            }else
                s.pop();
            prev = curr;
            if(s.size() > maxDepth)
                maxDepth = s.size();
        }
        return maxDepth;
    }
}

MaximumDepthOfBinaryTreeLevelOrder.java

public class Solution {
    public int maxDepthLevelOrder(TreeNode root){
    	if(root == null)
            return 0;
        int res = 1;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root);
        q.offer(null);
        while(!q.isEmpty()){
            TreeNode tmp = q.poll();
            if(tmp != null){
                if(tmp.left != null)
                    q.offer(tmp.left);
                if(tmp.right != null)
                    q.offer(tmp.right);
            }else{
                if(!q.isEmpty()){
                    ++res;
                    q.offer(null);
                }
            }
        }
        return res;
	}
}