/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init() { self.val = 0; self.left = nil; self.right = nil; }
 *     public init(_ val: Int) { self.val = val; self.left = nil; self.right = nil; }
 *     public init(_ val: Int, _ left: TreeNode?, _ right: TreeNode?) {
 *         self.val = val
 *         self.left = left
 *         self.right = right
 *     }
 * }
 
 
*/
class Solution {
    var res:[Int] = []
    func preorderTraversal(_ root: TreeNode?) -> [Int] {
        if root == nil {return []}
        res.append(root!.val)
        preorderTraversal(root!.left)
        preorderTraversal(root!.right)  
        return res
    }
}

概念

前序遍歷：根節點-左子樹-右子樹

中序遍歷：左子樹-根節點-右子樹

後序遍歷：左子樹-右子樹-根節點

層次遍歷：一層一層遍歷

前序遍歷：ABDECFG
中序遍歷：DBEAFCG
後序遍歷：DEBFGCA
層序遍歷：ABCDEFG