二叉树的遍历（循环及递归实现）——Java

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package go.jacob.day502;import java.util.Stack;/** *  * @author Administrator 前序遍历：6 3 1 2 5 4 9 7 8 中序遍历：1 2 3 4 5 6 7 8 9 后序遍历：2 1 *         4 5 3 8 7 9 6 */public class Demo3 {public static void main(String[] args) {Node root = init();System.out.println("|-----递归------|");System.out.print("前序遍历：");preOrderTraversal_rec(root);System.out.println();System.out.print("中序遍历：");inOrderTraversal_rec(root);System.out.println();System.out.print("后序遍历：");postOrderTraversal_rec(root);System.out.println();System.out.println("|-----循环------|");preOrderTraversal_cir(root);inOrderTraversal_cir(root);postOrderTraversal_cir(root);}/* * 递归实现：前序遍历 */private static void preOrderTraversal_rec(Node root) {if (root == null)return;System.out.print(root.val + " ");preOrderTraversal_rec(root.left);preOrderTraversal_rec(root.right);}/* * 递归实现：中序遍历 */private static void inOrderTraversal_rec(Node root) {if (root == null)return;inOrderTraversal_rec(root.left);System.out.print(root.val + " ");inOrderTraversal_rec(root.right);}/* * 递归实现：后续遍历 */private static void postOrderTraversal_rec(Node root) {if (root == null)return;postOrderTraversal_rec(root.left);postOrderTraversal_rec(root.right);System.out.print(root.val + " ");}/* * 循环实现：前序遍历 */private static void preOrderTraversal_cir(Node root) {if (root == null) {System.out.println("树为空");return;}Stack<Node> stack = new Stack<Node>();System.out.print("先序遍历：");while (root != null || !stack.isEmpty()) {if (root != null) {stack.push(root);System.out.print(root.val + " ");root = root.left;} else {root = stack.pop();root = root.right;}}System.out.println();}/* * 循环实现：中序遍历 */private static void inOrderTraversal_cir(Node root) {if (root == null) {System.out.println("树为空");return;}Stack<Node> stack = new Stack<Node>();System.out.print("中序遍历：");while (root != null || !stack.isEmpty()) {if (root != null) {stack.push(root);root = root.left;} else {root = stack.pop();System.out.print(root.val + " ");root = root.right;}}System.out.println();}/* * 循环实现：后续遍历 */private static void postOrderTraversal_cir(Node root) {if (root == null) {System.out.println("树为空");return;}System.out.print("后序遍历：");Stack<Node> stack = new Stack<Node>();Stack<Node> output = new Stack<Node>();while (root != null || !stack.isEmpty()) {if (root != null) {stack.push(root);output.push(root);root=root.right;}else{root=stack.pop();root=root.left;}}while(!output.isEmpty()){System.out.print(output.pop().val+" ");}System.out.println();}private static Node init() {Node J = new Node(8, null, null);Node H = new Node(4, null, null);Node G = new Node(2, null, null);Node F = new Node(7, null, J);Node E = new Node(5, H, null);Node D = new Node(1, null, G);Node C = new Node(9, F, null);Node B = new Node(3, D, E);Node A = new Node(6, B, C);return A; // 返回根节点}}class Node {int val = 0;Node left = null;Node right = null;public Node(int val, Node left, Node right) {this.val = val;this.left = left;this.right = right;}}
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