二叉树的花式遍历

1、前序遍历

递归：

public class preOrder_recursion {
    public static void preOrder(BinaryTreeNode root) {
        if (root == null) {
            return;
        }
        System.out.print(root.val + " ");
        preOrder(root.left);
        preOrder(root.right);
    }
}

迭代：

import java.util.Stack;

/**
 * Created by xzy on 2017/4/16.
 */
public class preOrder_iteration {
    public static void preOrder(BinaryTreeNode node) {
        if (node == null) {
            return;
        }
        Stack<BinaryTreeNode> st = new Stack<BinaryTreeNode>();

        while (true) {
            while (node != null) {
                System.out.print(node.val + " ");
                st.push(node);
                node = node.left;
            }
            if (st.empty()) {
                break;
            }
            node = st.pop().right;
        }
    }
}

2、中序遍历：

递归：

public class inOrder_recursion {
    public static void inOrder(BinaryTreeNode node){
        if (node == null) {
            return;
        }
        inOrder(node.left);
        System.out.print(node.val + " ");
        inOrder(node.right);
    }
}

迭代：

import java.util.Stack;

/**
 * Created by xzy on 2017/4/16.
 */
public class inOrder_iteration {
    public static void inOrder(BinaryTreeNode node) {
        if (node == null) {
            return;
        }
        Stack<BinaryTreeNode> st = new Stack<BinaryTreeNode>();
        while (true) {
            while (node != null) {
                st.push(node);
                node = node.left;
            }
            if (st.empty()) {
                break;
            }
            node = st.pop();
            System.out.print(node.val + " ");
            node = node.right;
        }
    }
}

3、后续遍历：

递归：

package treeOrder;

/**
 * Created by xzy on 2017/4/16.
 */
public class postOrder_recursion {
    public static void postOrder(BinaryTreeNode node) {
        if (node == null) {
            return;
        }
        postOrder(node.left);
        postOrder(node.right);
        System.out.print(node.val + " ");
    }
}

迭代：

import java.util.Stack;

/**
 * Created by xzy on 2017/4/16.
 */
public class postOrder_iteration {
    public static void postOrder(BinaryTreeNode node) {
        if (node == null) {
            return;
        }
        Stack<BinaryTreeNode> st = new Stack<BinaryTreeNode>();
        while (true) {
            while (node != null) {
                st.push(node);
                node = node.left;
            }
            while (!st.empty()) {
                if (node == st.peek().left && st.peek().right != null) {
                    node = st.peek().right;
                    break;
                } else {
                    node = st.pop();
                    System.out.print(node.val + " ");
                }
            }
            if (st.empty()) {
                break;
            }
        }
    }
}

4、层次遍历：

import java.util.LinkedList;

/**
 * Created by xzy on 2017/4/16.
 */
public class LevelOrder {
    public static void levelOrder(BinaryTreeNode node) {
        if (node == null) {
            return;
        }
        LinkedList<BinaryTreeNode> queue = new LinkedList<BinaryTreeNode>();
        queue.add(node);
        while (!queue.isEmpty()) {
            node = queue.poll();
            System.out.print(node.val + " ");
            if (node.left != null) {
                queue.add(node.left);
            }
            if (node.right != null) {
                queue.add(node.right);
            }
        }
    }
}