Thinking process I - Divide and Conquer

• what is the maximum depth of left subtree?
• what is the maximum depth of right subtree?
• get the bigger one and then plus 1, is the maximum depth at current node

O(n)

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int leftDepth = maxDepth(root.left);
        int rightDepth = maxDepth(root.right);

        return Math.max(leftDepth, rightDepth) + 1;
    }
}

Thinking process II - Traversal

Specifically, this is pre-order traversal

public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: An integer.
     */

    private int depth;
    public int maxDepth(TreeNode root) {
        depth = 0;
        dfsHelper(root, 0);
        return depth;
    }

    private void dfsHelper(TreeNode node, int currDepth) {
        if (node == null) {
            return;
        }

        currDepth += 1;
        depth = Math.max(currDepth, depth);

        dfsHelper(node.left, currDepth);
        dfsHelper(node.right, currDepth);
    }
}