104. Maximum Depth of Binary Tree

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
      if(root == null){
        return 0;
      }
      
      int left = maxDepth(root.left);
      int right = maxDepth(root.right);
      int current = 1 + Math.max(left, right);
      return current;
        
    }
}

class Solution {
    public int maxDepth(TreeNode root) {
        return helper(root);
        
    }
    private int helper(TreeNode root){
        if(root == null) return 0;
        
        // cur level
        
        int left = helper(root.left);
        int right = helper(root.right);
        return Math.max(left, right ) + 1;
    }
}

 

 

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.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.