二叉树11

以中序为例，每走一个节点就遍历一遍左子树，为空就返回，打印根节点，遍历右子树。
每个数字走3遍，数字打印的顺序（递归序）决定了是先序/中序/后序

先序中序后序遍历

public class TraversalBinaryTree {
    public static class Node{
        public int value;
        public Node left;
        public Node right;

        public Node(int v){
            value = v;
        }
    }
    public static void f(Node head){
        if(head==null){
            return; //如果节点为null,结束方法返回上一层
        }
        f(head.left); //遍历左节点
        f(head.right);//遍历右节点
    }
    //先序
    public static void pre(Node head) {
        if(head==null){
            return;
        }
        System.out.println(head.value);
        pre(head.left); //扫描左子树
        pre(head.right);//扫描右子树
    }
    //中序
    public static void in(Node head){
        if(head==null){
            return;
        }
        in(head.left);
        System.out.println(head.value);
        in(head.right);
    }
    //后序
    public static void pos(Node head){
        if(head==null){
            return;
        }
        pos(head.left);
        pos(head.right);
        System.out.println(head.value);
    }

    public static void main(String[] args) {
        Node head = new Node(1);
        head.left = new Node(2);
        head.right = new Node(3);
        head.left.left = new Node(4);
        head.left.right = new Node(5);
        head.right.left = new Node(6);
        head.right.right = new Node(7);
        pre(head);
        System.out.println("----------");
        in(head);
        System.out.println("----------");
        pos(head);
    }
}

判断两棵树结构是否相等
理解：遍历过程中始终都一样（同时为空）

//判断两颗树的结构是否相等
public class Struct {
    public static class TreeNode{
        public int val;
        public TreeNode left;
        public TreeNode right;
    }
    public static boolean isSameTree(TreeNode p,TreeNode q){
        //其中一棵树为空
        if(p == null ^ q == null){
            return false;
        }
        //两棵树都为空
        if(p==null && q == null){
            return true;
        }
        return p.val == q.val && isSameTree(p.left,q.left) && isSameTree(p.right,q.right);
    }
}

镜面树

头节点不会破坏镜面关系
左子树的左节点要和右子树的右节点一样
左子树的右节点要和右子树的左节点一样

public class Struct {
    public static class TreeNode{
        public int val;
        public TreeNode left;
        public TreeNode right;
    }
    public static boolean isSymmetric(TreeNode root){
        return isMirror(root,root);
    }

    private static boolean isMirror(TreeNode root, TreeNode root1) {
        if(root == null ^ root1 == null){
            return false;
        }
        if(root == null && root1 ==null){
            return true;
        }
        return root.val == root1.val && isMirror(root.left,root1.right) && isMirror(root.right,root1.left);
    }
}