二叉树的递归与非递归遍历

 二叉树节点类

public class BTNode {
    private char key;
    private BTNode left, right;

    public BTNode(char key) {
        this(key, null, null);
    }

    public BTNode(char key, BTNode left, BTNode right) {
        this.key = key;
        this.left = left;
        this.right = right;
    }

    public char getKey() {
        return key;
    }

    public void setKey(char key) {
        this.key = key;
    }

    public BTNode getLeft() {
        return left;
    }

    public void setLeft(BTNode left) {
        this.left = left;
    }

    public BTNode getRight() {
        return right;
    }

    public void setRight(BTNode right) {
        this.right = right;
    }
}

 遍历二叉树

/** 二叉树遍历 */
public class BinTree {
    protected BTNode root;

    public BinTree(BTNode root) {
        this.root = root;
    }

    public BTNode getRoot() {
        return root;
    }

    /** 构造树 */
    public static BTNode init() {
        BTNode a = new BTNode('A');
        BTNode b = new BTNode('B', null, a);
        BTNode c = new BTNode('C');
        BTNode d = new BTNode('D', b, c);
        BTNode e = new BTNode('E');
        BTNode f = new BTNode('F', e, null);
        BTNode g = new BTNode('G', null, f);
        BTNode h = new BTNode('H', d, g);
        return h;// root
    }

    /** 访问节点 */
    public static void visit(BTNode p) {
        System.out.print(p.getKey() + " ");
    }

    /** 递归实现前序遍历 */
    protected static void preorder(BTNode p) {
        if (p != null) {
            visit(p);
            preorder(p.getLeft());
            preorder(p.getRight());
        }
    }

    /** 递归实现中序遍历 */
    protected static void inorder(BTNode p) {
        if (p != null) {
            inorder(p.getLeft());
            visit(p);
            inorder(p.getRight());
        }
    }

    /** 递归实现后序遍历 */
    protected static void postorder(BTNode p) {
        if (p != null) {
            postorder(p.getLeft());
            postorder(p.getRight());
            visit(p);
        }
    }

    /** 非递归实现前序遍历 */
    protected static void iterativePreorder(BTNode p) {
        Stack<BTNode> stack = new Stack<BTNode>();
        if (p != null) {
            stack.push(p);
            while (!stack.empty()) {
                p = stack.pop();
                visit(p);
                if (p.getRight() != null)
                    stack.push(p.getRight());
                if (p.getLeft() != null)
                    stack.push(p.getLeft());
            }
        }
    }

    /** 非递归实现后序遍历 */
    protected static void iterativePostorder(BTNode p) {
        BTNode q = p;
        Stack<BTNode> stack = new Stack<BTNode>();
        while (p != null) {
            // 左子树入栈
            for (; p.getLeft() != null; p = p.getLeft())
                stack.push(p);
            // 当前节点无右子或右子已经输出
            while (p != null && (p.getRight() == null || p.getRight() == q)) {
                visit(p);
                q = p;// 记录上一个已输出节点
                if (stack.empty())
                    return;
                p = stack.pop();
            }
            // 处理右子
            stack.push(p);
            p = p.getRight();
        }
    }

    /** 非递归实现中序遍历 */
    protected static void iterativeInorder(BTNode p) {
        Stack<BTNode> stack = new Stack<BTNode>();
        while (p != null) {
            while (p != null) {
                if (p.getRight() != null)
                    stack.push(p.getRight());// 当前节点右子入栈
                stack.push(p);// 当前节点入栈
                p = p.getLeft();
            }
            p = stack.pop();
            while (!stack.empty() && p.getRight() == null) {
                visit(p);
                p = stack.pop();
            }
            visit(p);
            if (!stack.empty())
                p = stack.pop();
            else
                p = null;
        }
    }

    public static void main(String[] args) {
        BinTree tree = new BinTree(init());
        System.out.print(" Pre-Order:");
        preorder(tree.getRoot());
        System.out.println();
        System.out.print("  In-Order:");
        inorder(tree.getRoot());
        System.out.println();
        System.out.print("Post-Order:");
        postorder(tree.getRoot());
        System.out.println();
        System.out.print(" Pre-Order:");
        iterativePreorder(tree.getRoot());
        System.out.println();
        System.out.print("  In-Order:");
        iterativeInorder(tree.getRoot());
        System.out.println();
        System.out.print("Post-Order:");
        iterativePostorder(tree.getRoot());
        System.out.println();
    }
}

 输出结果
 Pre-Order:H D B A C G F E
  In-Order:B A D C H G E F
Post-Order:A B C D E F G H
 Pre-Order:H D B A C G F E
  In-Order:B A D C H G E F
Post-Order:A B C D E F G H