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;
}