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Prolem 104

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

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2
3
4
5
  3
 / \
9  20
  /  \
 15   7

return its depth = 3.

key

判断树的深浅，采用

int left = max(root.left);
int right = max(root.right);
return Math.max(left,right) + 1;

//或者简写

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

进行递归

Runtime: 0 ms, faster than 100.00% of Java online submissions for Maximum Depth of Binary Tree.

Memory Usage: 39.2 MB, less than 94.62% of Java online submissions for Maximum Depth of Binary Tree.

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