104. Maximum Depth of Binary Tree

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.

1. Depth-first-search

Only one line code.

int maxDepth(TreeNode *root){    return root == NULL ? 0 : max(maxDepth(root -> left), maxDepth(root -> right)) + 1;}

2. Breadth-first-search

Calculate the count of the last level.

int maxDepth(TreeNode *root){    if(root == NULL)        return 0;    int res = 0;    queue<TreeNode *> q;    q.push(root);    while(!q.empty())    {        ++ res;        for(int i = 0, n = q.size(); i < n; ++ i)        {            TreeNode *p = q.front();            q.pop();            if(p -> left != NULL)                q.push(p -> left);            if(p -> right != NULL)                q.push(p -> right);        }    }    return res;}