思路：递归，DFS或者BFS层次遍历
递归版本

class Solution {
    public int maxDepth(TreeNode root) {
        if (root == null) {
            return 0;
        } else {
        	// 左子树高度
            int leftHeight = maxDepth(root.left);
            // 右子树高度
            int rightHeight = maxDepth(root.right);
            // 返回高度较高的子树+1
            return Math.max(leftHeight, rightHeight) + 1;
        }
    }
}

DFS

class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null){
            return 0;
        }
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);

        return Math.max(left+1, right+1);
    }
}

BFS层次遍历

class Solution {
    public int maxDepth(TreeNode root) {
        LinkedList<TreeNode> queue = new LinkedList<>();
        if (root == null)   return 0;
        queue.add(root);
        int depth = 0;
        while (!queue.isEmpty()){
            depth++;
            int size = queue.size();
            // 注意不要for (int i = 0; i < queue.size(); i++)，因为queue.size()会变化
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                if (node.left != null)  queue.add(node.left);
                if (node.right != null) queue.add(node.right);
            }
        }
        return depth;
    }
}