Leecode104 二叉树的最大深度

//DFS解法 前序遍历
/**
 * 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) {
        if(root == null) {
            return 0;
        }
        int rightDepth = maxDepth(root.right);
        int leftDepth = maxDepth(root.left);
        return 1  + max(rightDepth,leftDepth);
    }
    public int max(int rightDepth, int leftDepth) {
        return rightDepth > leftDepth ? rightDepth :leftDepth;
    }
}

//BFS解法 层次遍历
/**
 * 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) {
      if(root == null) {return 0;}
      Queue<TreeNode> que = new LinkedList<>();
      que.offer(root);
      int depth = 0;
      while (!que.isEmpty()){
          int len  = que.size();
          depth ++;
          for (int i = 0; i < len; i++) {
              TreeNode node = que.poll();
              if(node.left != null) {
                  que.offer(node.left);
              }
              if(node.right != null) {
                  que.offer(node.right);
              }
          }
      }
      return depth;
    }
}