[Sword refers to Offer learning] [interview question 59: symmetric Binary Tree], sword refers to offer

[Sword refers to Offer learning] [interview question 59: symmetric Binary Tree], sword refers to offer
Question: implement a function to determine whether a binary tree is symmetric. If a binary tree is the same as its image, it is symmetric.Solutions

We usually have three different binary tree traversal algorithms, namely pre-order traversal, middle-order traversal, and post-order traversal. Among the three Traversal Algorithms, the system first traverses the left subnode and then the right subnode. Can we define a traversal algorithm that first traverses the right child node and then the left child node? For example, we define a symmetric Traversal Algorithm for pre-order traversal, that is, first traversing the parent node, then traversing its right child node, and finally traversing its left child node.
We found that we can use the pre-order traversal sequence of the Binary Tree and the symmetric pre-order traversal sequence to determine whether the binary tree is symmetric. If the two sequences are the same, the binary tree is symmetric.

Node Definition

private static class BinaryTreeNode {    private int val;    private BinaryTreeNode left;    private BinaryTreeNode right;    public BinaryTreeNode() {    }    public BinaryTreeNode(int val) {        this.val = val;    }    @Override    public String toString() {        return val + "";    }}

Code Implementation

public class Test59 {    private static class BinaryTreeNode {        private int val;        private BinaryTreeNode left;        private BinaryTreeNode right;        public BinaryTreeNode() {        }        public BinaryTreeNode(int val) {            this.val = val;        }        @Override        public String toString() {            return val + "";        }    }    public static boolean isSymmetrical(BinaryTreeNode root) {        return isSymmetrical(root, root);    }    private static boolean isSymmetrical(BinaryTreeNode left, BinaryTreeNode right) {        if (left == null && right == null) {            return true;        }        if (left == null || right == null) {            return false;        }        if (left.val != right.val ) {            return false;        }        return isSymmetrical(left.left, right.right) && isSymmetrical(left.right, right.left);    }    public static void main(String[] args) {        test01();        test02();    }    private static void assemble(BinaryTreeNode node,                                 BinaryTreeNode left,                                 BinaryTreeNode right) {        node.left = left;        node.right = right;    }    //                            1    //                  2                   2    //             4         6          6          4    //          8     9   10   11   11     10   9     8    public static void test01() {        BinaryTreeNode n1 = new BinaryTreeNode(1);        BinaryTreeNode n2 = new BinaryTreeNode(2);        BinaryTreeNode n3 = new BinaryTreeNode(2);        BinaryTreeNode n4 = new BinaryTreeNode(4);        BinaryTreeNode n5 = new BinaryTreeNode(6);        BinaryTreeNode n6 = new BinaryTreeNode(6);        BinaryTreeNode n7 = new BinaryTreeNode(4);        BinaryTreeNode n8 = new BinaryTreeNode(8);        BinaryTreeNode n9 = new BinaryTreeNode(9);        BinaryTreeNode n10 = new BinaryTreeNode(10);        BinaryTreeNode n11 = new BinaryTreeNode(11);        BinaryTreeNode n12 = new BinaryTreeNode(11);        BinaryTreeNode n13 = new BinaryTreeNode(10);        BinaryTreeNode n14 = new BinaryTreeNode(9);        BinaryTreeNode n15 = new BinaryTreeNode(8);        assemble(n1, n2, n3);        assemble(n2, n4, n5);        assemble(n3, n6, n7);        assemble(n4, n8, n9);        assemble(n5, n10, n11);        assemble(n6, n12, n13);        assemble(n7, n14, n15);        assemble(n8, null, null);        assemble(n9, null, null);        assemble(n10, null, null);        assemble(n11, null, null);        assemble(n12, null, null);        assemble(n13, null, null);        assemble(n14, null, null);        assemble(n15, null, null);        System.out.println(isSymmetrical(n1));    }    //                            1    //                  2                   2    //             4         5          6          4    //          8     9   10   11   11     10   9     8    public static void test02() {        BinaryTreeNode n1 = new BinaryTreeNode(1);        BinaryTreeNode n2 = new BinaryTreeNode(2);        BinaryTreeNode n3 = new BinaryTreeNode(2);        BinaryTreeNode n4 = new BinaryTreeNode(4);        BinaryTreeNode n5 = new BinaryTreeNode(5);        BinaryTreeNode n6 = new BinaryTreeNode(6);        BinaryTreeNode n7 = new BinaryTreeNode(4);        BinaryTreeNode n8 = new BinaryTreeNode(8);        BinaryTreeNode n9 = new BinaryTreeNode(9);        BinaryTreeNode n10 = new BinaryTreeNode(10);        BinaryTreeNode n11 = new BinaryTreeNode(11);        BinaryTreeNode n12 = new BinaryTreeNode(11);        BinaryTreeNode n13 = new BinaryTreeNode(10);        BinaryTreeNode n14 = new BinaryTreeNode(9);        BinaryTreeNode n15 = new BinaryTreeNode(8);        assemble(n1, n2, n3);        assemble(n2, n4, n5);        assemble(n3, n6, n7);        assemble(n4, n8, n9);        assemble(n5, n10, n11);        assemble(n6, n12, n13);        assemble(n7, n14, n15);        assemble(n8, null, null);        assemble(n9, null, null);        assemble(n10, null, null);        assemble(n11, null, null);        assemble(n12, null, null);        assemble(n13, null, null);        assemble(n14, null, null);        assemble(n15, null, null);        System.out.println(isSymmetrical(n1));    }}

Running result

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