leetcode-144-binary-tree-preorder-traversal.swift

/**

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https://leetcode.com/problems/binary-tree-preorder-traversal/


Given a binary tree, return the preorder traversal of its nodes' values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]
Follow up: Recursive solution is trivial, could you do it iteratively?

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of nodes in root.
Space complexity O(n) where n is the amount of nodes in root.
*/
func inorderTraversal(_ root: TreeNode?) -> [Int] {
        
    var result: [Int] = []
    root?.traversePostOrder { result.append($0) }
    return result
}

extension TreeNode {

    /**
    Will be called recursively 
    until the whole tree is traversed.
    */
    func traversePreOrder(_ visit: (Int) -> Void) {

        // Pre order starts at the current node
        visit(val)

        // Then moves down the left branch
        left?.traversePreOrder(visit)

        // And lastly the right branch
        right?.traversePreOrder(visit)
    }
}