leetcode-104-maximum-depth-of-binary-tree.swift

/**

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https://leetcode.com/problems/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.

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of nodes in root.
Space complexity O(1) we're not using any extra space.
*/
func maxDepth(_ root: TreeNode?) -> Int {
        
    // Init a 0 depth
    var depth: Int = 0 
    
    /**
    Traverse through the whole tree
    and get the depth of every node
    */
    traverse(root) { level in

        /**
        Compare to the current deepest depth 
        and only keep the depeest one we have seen
        */
        depth = max(depth, level)
    }
    
    return depth
}

/**
Take in a node and a current depth level
and in the close returns the depth level of every node.

Will traverse the whole tree and be called recursively
until there is no more node left.
*/
func traverse(_ node: TreeNode?, at level: Int = 1, visited: (Int) -> Void) {
    
    guard let node = node else  {return }
    visited(level)
    traverse(node.left, at: level + 1, visited: visited)
    traverse(node.right, at: level + 1, visited: visited)
}