剑指 Offer 55 - I. 二叉树的深度

解法一：广度优先遍历

层序遍历。

/**
 * 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) {
        Queue<TreeNode> queue = new LinkedList<>();
        if (root != null) {
            queue.offer(root);
        }
        int depth = 0;
        while (!queue.isEmpty()) {
            ++depth;
            int size = queue.size();
            for (int i = 0; i < size; ++i) {
                TreeNode temp = queue.poll();
                if (temp.left != null) {
                    queue.offer(temp.left);
                }
                if (temp.right != null) {
                    queue.offer(temp.right);
                }
            }
        }
        return depth;
    }
}

解法二：深度优先遍历

每棵树的深度相当于左右子树深度较大值+1，相当简洁。

/**
 * 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) {
        return dfs(root);
    }
    private int dfs(TreeNode root) {
        if (root == null) {
            return 0;
        }
        return Math.max(dfs(root.left) + 1, dfs(root.right) + 1);
    }
}