java树的遍历非递归算法 java树的遍历三种顺序

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云端梦想实现家 2023-08-21 12:36:43

文章标签 java树的遍历非递归算法 System Stack 子树 文章分类 Java 后端开发

1. 遍历算法（遍历二叉树6种方法）
1.1. 概述
遍历算法针对二叉树而言的，主要有先序、中序、后序三种遍历顺序，三种顺序又分别有递归和常规算法，二叉树遍历的主要思想是：遍历左子树，遍历右子树，访问根节点，由这三者的遍历顺序来确定是先序、中序还是后序。下面只要求掌握递归遍历算法，常规遍历算法见附录一。
1.2. 先序遍历算法
遍历顺序：访问根节点，遍历左子树，遍历右子树。代码如下：

void preOrder(BinaryTreeNode bt) {   if (bt == null)// 如果当前树为空，则终止递归    return;   System.out.print(bt.getData());// 先访问根节点   
 preOrder(bt.getLeftChild());// 再遍历左子树 
   preOrder(bt.getRightChild());// 再遍历右子树 
  }

1.3. 中序遍历算法
遍历顺序：遍历左子树，访问根节点，遍历右子树。代码如下：

void midOrder(BinaryTreeNode bt) { 
  if (bt == null)// 如果当前树为空，则终止递归   
 return;   
preOrder(bt.getLeftChild());// 先遍历左子树   
System.out.print(bt.getData());// 再访问根节点  
 preOrder(bt.getRightChild());// 再遍历右子树
  }

1.4. 后序遍历算法

遍历顺序：遍历左子树，遍历右子树，访问根节点。代码如下：

void postOrder(BinaryTreeNode bt) {   

if (bt == null)// 如果当前树为空，则终止递归   
 return;  
 preOrder(bt.getLeftChild());// 先遍历左子树  
 preOrder(bt.getRightChild());// 再遍历右子树 

   System.out.print(bt.getData());// 再访问根节点 

  }

1.5. 层次遍历算法

void levelOrder(BinaryTreeNode bt) { 

    if (bt == null)     

 return; 

    Queue q = new ArrayQueue();    q.enqueue(bt); 

   while (!q.isEmpty()) {     bt = (BinaryTreeNode) q.dequeue();// 取出队首元素，访问之    
System.out.println(bt.getData());    
 if (bt.hasLeftChild()) {
     q.enqueue(bt.getLeftChild());// 如果左节点存在，放入队列中    
 } 
     if (bt.hasRightChild()) { 

     q.enqueue(bt.getRightChild());// 如果右节点存在，放入队列中     
} 
    } 

   

   

}

package edu.cumt.jnotnull;

import java.util.Stack;

public class BinaryTree {
	protected Node root;

	public BinaryTree(Node root) {
		this.root = root;
	}

	public Node getRoot() {
		return root;
	}

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

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

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

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

	/** 递归实现后序遍历 */
	protected static void postorder(Node 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(Node p) {
		Stack<Node> stack = new Stack<Node>();
		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());
			}
		}
	}

	/** 非递归实现前序遍历2 */
	protected static void iterativePreorder2(Node p) {
		Stack<Node> stack = new Stack<Node>();
		Node node = p;
		while (node != null || stack.size() > 0) {
			while (node != null) {//压入所有的左节点，压入前访问它
				visit(node);
				stack.push(node);
				node = node.getLeft();
			}
			if (stack.size() > 0) {//
				node = stack.pop();
				node = node.getRight();
			}
		}
	}

	/** 非递归实现后序遍历 */
	protected static void iterativePostorder(Node p) {
		Node q = p;
		Stack<Node> stack = new Stack<Node>();
		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();
		}
	}

	/** 非递归实现后序遍历 双栈法 */
	protected static void iterativePostorder2(Node p) {
		Stack<Node> lstack = new Stack<Node>();
		Stack<Node> rstack = new Stack<Node>();
		Node node = p, right;
		do {
			while (node != null) {
				right = node.getRight();
				lstack.push(node);
				rstack.push(right);
				node = node.getLeft();
			}
			node = lstack.pop();
			right = rstack.pop();
			if (right == null) {
				visit(node);
			} else {
				lstack.push(node);
				rstack.push(null);
			}
			node = right;
		} while (lstack.size() > 0 || rstack.size() > 0);
	}

	/** 非递归实现后序遍历 单栈法*/
	protected static void iterativePostorder3(Node p) {
		Stack<Node> stack = new Stack<Node>();
		Node node = p, prev = p;
		while (node != null || stack.size() > 0) {
			while (node != null) {
				stack.push(node);
				node = node.getLeft();
			}
			if (stack.size() > 0) {
				Node temp = stack.peek().getRight();
				if (temp == null || temp == prev) {
					node = stack.pop();
					visit(node);
					prev = node;
					node = null;
				} else {
					node = temp;
				}
			}

		}
	}

	/** 非递归实现后序遍历4 双栈法*/
	protected static void iterativePostorder4(Node p) {
		Stack<Node> stack = new Stack<Node>();
		Stack<Node> temp = new Stack<Node>();
		Node node = p;
		while (node != null || stack.size() > 0) {
			while (node != null) {
				temp.push(node);
				stack.push(node);
				node = node.getRight();
			}
			if (stack.size() > 0) {
				node = stack.pop();
				node = node.getLeft();
			}
		}
		while (temp.size() > 0) {
			node = temp.pop();
			visit(node);
		}
	}

	/** 非递归实现中序遍历 */
	protected static void iterativeInorder(Node p) {
		Stack<Node> stack = new Stack<Node>();
		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;
		}
	}

	/** 非递归实现中序遍历2 */
	protected static void iterativeInorder2(Node p) {
		Stack<Node> stack = new Stack<Node>();
		Node node = p;
		while (node != null || stack.size() > 0) {
			while (node != null) {
				stack.push(node);
				node = node.getLeft();
			}
			if (stack.size() > 0) {
				node = stack.pop();
				visit(node);
				node = node.getRight();
			}
		}
	}

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		BinaryTree tree = new BinaryTree(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("Pre-Order2:");
		iterativePreorder2(tree.getRoot());
		System.out.println();
		System.out.print("  In-Order:");
		iterativeInorder(tree.getRoot());
		System.out.println();
		System.out.print(" In-Order2:");
		iterativeInorder2(tree.getRoot());
		System.out.println();
		System.out.print(" Post-Order:");
		iterativePostorder(tree.getRoot());
		System.out.println();
		System.out.print("Post-Order2:");
		iterativePostorder2(tree.getRoot());
		System.out.println();
		System.out.print("Post-Order3:");
		iterativePostorder3(tree.getRoot());
		System.out.println();
		System.out.print("Post-Order4:");
		iterativePostorder4(tree.getRoot());
		System.out.println();

	}

}

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