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.

Note: A leaf is a node with no children.

Example:

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

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.

Solution: Recursion¶

maxDepth(root) = max(maxDepth(root.left), maxDepth(root.right)) + 1

Time complexity: O(n)

Space complexity: O(n)

class Solution {
public:
  int maxDepth(TreeNode* root) {
    if (!root) return 0;
    int l = maxDepth(root->left);
    int r = maxDepth(root->right);
    return max(l, r) + 1;
  }
};