二叉树的遍历

二叉树的四种遍历方式：前序遍历、中序遍历、后序遍历和层序遍历

package binaryTree;
public class TreeNode {
	int val;
	TreeNode left;
	TreeNode right;

	TreeNode(int x) {
		val = x;
	}
}

package binaryTree;

import java.util.ArrayList;
import java.util.LinkedList;

public class BinaryTreeTraverse {

	public static void main(String[] args) {
		TreeNode root = new TreeNode(0);
		TreeNode node1 = new TreeNode(1);
		TreeNode node2 = new TreeNode(2);
		TreeNode node3 = new TreeNode(3);
		TreeNode node4 = new TreeNode(4);
		TreeNode node5 = new TreeNode(5);
		TreeNode node6 = new TreeNode(6);

		root.left = node1;
		root.right = node2;
		node1.left = node3;
		node1.right = node4;
		node2.left = node5;
		node2.right = node6;
		// 辅助队列
		LinkedList<TreeNode> helper = new LinkedList<TreeNode>();
		ArrayList<Integer> list  = new ArrayList<Integer>();
		
		preOrderTraverse(root,list);
		System.out.println("前序遍历"+list);
		list.clear();
		inOrderTraverse(root,list);
		System.out.println("中序遍历"+list);
		list.clear();
		postOrderTraverse(root,list);
		System.out.println("后序遍历"+list);
		list.clear();
		if(root!=null) {
			helper.add(root);
			floorOrderTraver(helper,list);
		}
		System.out.println("层序遍历"+list);
		list.clear();
		helper.clear();

	}

	// 前序遍历；
	private static void preOrderTraverse(TreeNode root, ArrayList<Integer> list) {
		if (root == null) {
			return;
		}
		list.add(root.val);
		preOrderTraverse(root.left, list);
		preOrderTraverse(root.right, list);
	}

	// 中序遍历；
	private static void inOrderTraverse(TreeNode root, ArrayList<Integer> list) {
		if (root == null) {
			return;
		}
		inOrderTraverse(root.left, list);
		list.add(root.val);
		inOrderTraverse(root.right, list);
	}

	// 后序遍历；
	private static void postOrderTraverse(TreeNode root, ArrayList<Integer> list) {
		if (root == null) {
			return;
		}
		postOrderTraverse(root.left, list);
		postOrderTraverse(root.right, list);
		list.add(root.val);
	}

	// 层序遍历
	private static void floorOrderTraver(LinkedList<TreeNode> helper, ArrayList<Integer> list) {
		while (!helper.isEmpty()) {
			// 从队列中移除并返回；
			TreeNode tmp = helper.pollFirst();
			list.add(tmp.val);
			if (tmp.left != null) {
				helper.offerLast(tmp.left);
			}
			if (tmp.right != null) {
				helper.offerLast(tmp.right);
			}
		}

	}

}

运行结果