preOrder: (root) LLL RRRR
inOrder: LLL (root) RRRR
inOrder: LLL RRRR (root)
根据preOrder/postOrder 可以知道root(一头一尾); 然后找到root在inorder中对应的位置（index）
index左边即为Left Branch(inorder), 右边是Right Branch(inorder). 确定LeftBranch && RightBranch的长度后，同样可以从PreOrder/PostOrder中获得Left Branch(preorder/postorder) && Right Branch(preorder/postorder)
在分别对leftBranch && right Branch 进行recursive call.
Depth-First Transversal (3 types)
preorder: root --> preorder(root.child) --> preorder(root.right)
inOrder: recursive call on leftBranch --> root --> recursive call on rightBranch
postorder: recursive call on leftBranch--> recursive call on rightBranch-->root
=>Naming regarding the position of root !
//pre-Condition: root is a valid treeNode
preorder(Node){
//boundary case: Node is a leaf
//recursive case: Node has child(s)
1. do something to root
2. if(root.left != null)
preorder(root.left)
3. if(root.right != null)
preorder(root.right)
}