104.java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public int maxDepth(TreeNode root) {
        //.  1 
        //  / \    
        // 2   3
        
        // ROOT -> LEAF: top to bottom
        
        // Post-order: LEFT, RIGHT, ROOT: bottom to top
        // [9], [15, 7, 20], 3
        
        // base case
        if (root == null) return 0; // null node does not contribute to the depth of the tree
        
        // max depth of subTree using root.left as root
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        
        // visit the current node(root)
        // consider what to do for the current node, given the result of left subTree and right subTree
        return 1 + Math.max(left, right);
    }
}