Problem

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.

Pre analysis

Will take a recursive approach to solve this. Will find max of depth of left and right.

Another solution

Using DFS

var maxDepth = function(root) {
    let max = 0;

  function dfs(node, depth) {
    if (!node) max = Math.max(max, depth);
    else {
      dfs(node.left, depth + 1);
      dfs(node.right, depth + 1);
    }
  }

  dfs(root, max);
  return max;
};