基础知识：二叉树的遍历

class TreeNode {
    private Object element;
    private TreeNode leftChild;
    private TreeNode rightChild;

    //preOrder traversal recursion style
     public void preOrder(TreeNode root) {
        if (root != null) {
            System.out.println(root.element);
            preOrder(root.leftChild);
            preOrder(root.rightChild);
        }
    }

    //preOrder traversal no-recursion style
     public void preOrderNR(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        while (p != null || !stack.empty()) {

            // start from left-part until arrive leaf node
            // then pop the top item in stack to traversal the
            // right-part
            while (p != null) {
                System.out.println(p.element);
                stack.push(p);
                p = p.leftChild;
            }

            // when left-traversal finished pop top item in stack
            // traversal right-part
            if (!stack.empty()) {
                p = stack.pop();
                p = p.rightChild;
            }
        }
    }

    //inOrder traversal recursion style
    public void inOrder(TreeNode root) {
         if (root != null) {
             inOrder(root.leftChild);
             System.out.println(root.element);
             inOrder(rightChild);
         }
    }

    //inOrder traversal no-recursion style
    // little different with preOrder
    // just the step to read the root element
    public void inOrderNR(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        while (p != null || !stack.empty()) {
            while (p != null) {
                stack.push(p);
                p = p.leftChild;
            }

            if (!stack.empty()) {
                p = stack.pop();
                System.out.println(p.element);
                p = p.rightChild;
            }
        }
    }

    //postOrder traversal recursion style
    public void postOrder(TreeNode root) {
        if (root != null) {
            inOrder(root.leftChild);
            inOrder(rightChild);
            System.out.println(root.element);
        }
    }

    //postOrder traversal no-recursion style
    // when return from child should know it is which child
    // if arrive left-child just add ot in stack
    // else get top item in stack , if its right-child was visited
    // just print top item element because all left-child were visited
    public void postOrderNR(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode p = root;
        TreeNode flag = null;
        while (p != null || !stack.empty()) {
            if (p != null) {
                stack.push(p);
                p = p.leftChild;
            } else {
                p = stack.peek();
                if (p.rightChild != null && p.rightChild != flag) {
                    p = p.rightChild;
                } else {
                    stack.pop();
                    System.out.println(p.element);
                    flag = p;
                    p = null;
                }
            }
        }
    }
}