二叉树结构与递归实现前中后序遍历

1. 二叉树结构

2. 二叉树节点遍历顺序

前序:

每颗子树以 中 —》左 —》右 遍历

A B D E H C F G

中序:

每颗子树以 左 —》中 —》右 遍历

D B E H A F C G

后序:

每颗子树以 左 —》右 —》中 遍历

D H E B F G C A

代码实现:

public class BinaryTree {

    static class TreeNode {
        public TreeNode left;
        public char val;
        public TreeNode right;

        public TreeNode(char val) {
            this.val = val;
        }
    }

    public TreeNode createTree() {
        TreeNode A = new TreeNode('A');
        TreeNode B = new TreeNode('B');
        TreeNode C = new TreeNode('C');
        TreeNode D = new TreeNode('D');
        TreeNode E = new TreeNode('E');
        TreeNode F = new TreeNode('F');
        TreeNode G = new TreeNode('G');
        TreeNode H = new TreeNode('H');

        A.left = B;
        B.left = D;
        B.right = E;
        E.right = H;
        A.right = C;
        C.left = F;
        C.right = G;
        return A;
    }

    // 前序 - 根左右
    public void preOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        System.out.print(root.val + " ");
        preOrder(root.left);
        preOrder(root.right);
    }

    // 中序
    public void inOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        inOrder(root.left);
        System.out.print(root.val + " ");
        inOrder(root.right);
    }

    // 后序
    public void postOrder(TreeNode root) {
        if (root == null) {
            return;
        }
        postOrder(root.left);
        postOrder(root.right);
        System.out.print(root.val + " ");
    }

    public static void main(String[] args) {

        BinaryTree binaryTree = new BinaryTree();
        TreeNode root = binaryTree.createTree();

        binaryTree.preOrder(root);
        System.out.println();

        binaryTree.inOrder(root);
        System.out.println();

        binaryTree.postOrder(root);

    }

}

3. 递归序理解二叉树遍历

除了空节点外 每一个节点一定会到达3次

第一次到达访问节点: 前序，第二次到达访问节点: 中序，第三次到达访问节点: 后序

访问节点的时机不同 遍历顺序也就不同

以递归序为例: