Sword finger offer:55 - I. depth of binary tree

1. subject

​ ​ The finger of the sword Offer 55 - I. The depth of the binary tree ​​

2. describe

Enter the root node of a binary tree , Find the depth of the tree . The nodes that pass from the root node to the leaf node （ Containing root 、 Leaf nodes ） A path to a tree , The length of the longest path is the depth of the tree .

for example ：

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

3

/ \

9 20

/ \

15 7

Return to its maximum depth 3 .

Tips ：

Total number of nodes <= 10000

3. Implementation method

3.1 Method 1

3.1.1 Ideas

1. 1. 
      
2. First determine whether the root node is ​​null​​, Yes, the depth is 0;
3. Then traverse the larger depth in the left and right subtrees , And then add 1 （ The root node ） Is the depth of the binary tree ;
4. Because you need to traverse all nodes , So the time complexity is O(n);
   1. 
      

3.1.2 Realization

Log in and copy

       

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

        
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