树的遍历

Posted 2021-05-16 Shall潇

树的遍历：广度遍历和深度遍历

广度遍历：即层次遍历：一层一层遍历

深度遍历：

先序：双亲节点->左子树->右子树
中序：左子树->双亲节点->右子树
后序：左子树->右子树->双亲节点

结果

先序：1 2 4 6 7 8 3 5
中序：4 7 6 8 2 1 3 5
后序：7 8 6 4 2 5 3 1

实现源码

import java.util.Stack;

/**
 * @Author shall潇
 * @Date 2021/5/8
 * @Description 树的遍历：广度遍历，深度遍历
 *
 * 广度遍历：即层次遍历：一层一层遍历
 *
 * 深度遍历：
 * 先序：双亲节点->左子树->右子树
 * 中序：左子树->双亲节点->右子树
 * 后序：左子树->右子树->双亲节点
 */

class TreeNode{
    String date;
    TreeNode leftchlid;  //左孩子
    TreeNode rightchild; //右孩子
}

public class TraverableTree {
    /*-------------------------递归--------------------------*/
    //先序遍历
    public static void recursionPreorderTraversal(TreeNode root){
        if(root!=null){
            System.out.println(root.date);
            recursionPreorderTraversal(root.leftchlid);
            recursionPreorderTraversal(root.rightchild);
        }
    }
    //中序遍历
    public static void recursionMiddleorderTraversal(TreeNode root){
        if(root!=null){
            recursionMiddleorderTraversal(root.leftchlid);
            System.out.println(root.date);
            recursionMiddleorderTraversal(root.rightchild);
        }
    }
    //后序遍历
    public static void recursionPostorderTraversal(TreeNode root){
        if(root!=null){
            recursionPostorderTraversal(root.leftchlid);
            recursionPostorderTraversal(root.rightchild);
            System.out.println(root.date);
        }
    }
    /*-------------------------非递归--------------------------*/
    //先序
    public static void rePreorderTravseral(TreeNode root){
        Stack<TreeNode> treeNodesStack = new Stack<>();  //用栈来暂存当前节点
        TreeNode node = root;                            //保存原来数据
        while (node!=null || !treeNodesStack.isEmpty()){ //节点非空或者栈非空的话
            while (node!=null){
                System.out.println(node.date);           //首先打印节点数据
                treeNodesStack.push(node);               //把这个节点入栈
                node = node.leftchlid;                   //取出它的左子树
            }
            if(!treeNodesStack.isEmpty()){               //没有左子树了，该遍历右子树
                node = treeNodesStack.pop();             //先弹出栈
                node= node.rightchild;                   //遍历右子树
            }
        }
    }
    //中序
    public static void reMiddleorderTravseral(TreeNode root){
        Stack<TreeNode> treeNodeStack = new Stack<>();
        TreeNode node = root;
        while (node !=null || !treeNodeStack.isEmpty()){
            while (node!=null) {
                treeNodeStack.push(node);
                node = node.leftchlid;
            }
            if(!treeNodeStack.isEmpty()){
                node = treeNodeStack.pop();
                System.out.println(node.date);
                node = node.rightchild;
            }
        }
    }
    //后序
    public static void rePostorderTravseral(TreeNode root){
        Stack<TreeNode> treeNodeStack = new Stack<>();
        TreeNode node = root;
        TreeNode lastVisit = root;
        while (node!=null || !treeNodeStack.isEmpty()){
            while (node !=null){
                treeNodeStack.push(node);
                node = node.leftchlid;
            }
            node = treeNodeStack.peek();  	    //查看栈顶元素
            if(node.rightchild == null || node.rightchild == lastVisit){
                System.out.println(node.date); //如果右子树为空 或 右子树已经被访问过，直接输出
                treeNodeStack.pop();
                lastVisit = node;
                node = null;
            }else {                      	  //如果右子树没被访问或不为空，则遍历它
                node = node.rightchild;
            }
        }
    }
}