面试题：二叉树的前中后序遍历
二叉树的前中后遍历，其前中后，您可理解为指的是根结点所在的序。
前序遍历：前序遍历的顺序为根-左-右中序遍历：中序遍历的顺序为左-根-右后序遍历：后序遍历的顺序为左-右-根层次遍历: 从上到下，每一个层次从左到右
前序遍历
import java.util.ArrayList;import java.util.List;import java.util.Stack;/** * Given a binary tree, return the preorder traversal of its TreeNodes' values. */public class Lc144 { /* * 前序遍历 ：根左右 思路；将当前节点压入栈中，一直遍历左子树知道当前节点为空，向上弹出遍历一下右子树。 */ public static List<Integer> preorderTraversal(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); List<Integer> list = new ArrayList<>(); TreeNode curr = root; while (curr != null || !stack.isEmpty()) { while (curr != null) { list.add(curr.val); stack.push(curr); curr = curr.left; } curr = stack.pop(); curr = curr.right; } return list; } public static void main(String[] args) { Integer[] arr = new Integer[] { 1, null, 2, null, null, 3 }; TreeNode root = CreateNode.createTree(arr).get(0); List<Integer> list = preorderTraversal(root); list.forEach(n -> System.out.println(n)); }}
中序遍历
import java.util.ArrayList;import java.util.List;import java.util.Stack;/** * Given a binary tree, return the inorder traversal of its nodes' values. */public class Lc94 { /* * 中序遍历：左根右 */ public static List<Integer> inOrderTraversal(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); List<Integer> preorder = new ArrayList<Integer>(); TreeNode curr = root; while (!stack.isEmpty() || curr != null) { while (curr != null) { stack.push(curr); curr = curr.left; } curr = stack.pop(); preorder.add(curr.val); curr = curr.right; } return preorder; } public static void main(String[] args) { Integer[] arr = new Integer[] { 1, null, 2, null, null, 3 }; TreeNode root = CreateNode.createTree(arr).get(0); List<Integer> list = inOrderTraversal(root); list.forEach(n -> System.out.println(n)); }}
后续遍历
import java.util.ArrayList;import java.util.List;import java.util.Stack;public class Lc145 { /* * 后续序遍历：左右根 */ public static List<Integer> postorderTraversal(TreeNode root) { Stack<TreeNode> stack = new Stack<>(); List<Integer> list = new ArrayList<>(); if (root == null) { return list; } TreeNode curr = root; stack.push(curr); while (!stack.isEmpty()) { curr = stack.pop(); list.add(0, curr.val); if (curr.left != null) { stack.push(curr.left); } if (curr.right != null) { stack.push(curr.right); } } return list; } public static void main(String[] args) { Integer[] arr = new Integer[] { 0, 1, 2, 3, 4, 5, 6 }; TreeNode root = CreateNode.createTree(arr).get(0); List<Integer> list = postorderTraversal(root); list.forEach(n -> System.out.println(n)); }}
层序遍历
import java.util.ArrayList;import java.util.LinkedList;import java.util.List;import java.util.Queue;public class Lc102 { /** * 层序遍历 * @param root * @return */ public static List<List<Integer>> levelOrder(TreeNode root) { Queue<TreeNode> queue = new LinkedList<>(); List<List<Integer>> lists = new ArrayList<List<Integer>>(); List<Integer> list = new ArrayList<>(); if (root == null) { return lists; } TreeNode curr = root; queue.offer(curr); int size = queue.size(); while (!queue.isEmpty()) { curr = queue.poll(); list.add(curr.val); size--; if (curr.left != null) { queue.offer(curr.left); } if (curr.right != null) { queue.offer(curr.right); } if (size == 0) { lists.add(list); list = new ArrayList<>(); size = queue.size(); } } return lists; } public static void main(String[] args) { Integer[] arr = new Integer[] { 1, null, 2 }; TreeNode root = CreateNode.createTree(arr).get(0); List<List<Integer>> lists = levelOrder(root); lists.forEach(n -> { n.forEach(m -> { System.out.print(m + ","); }); System.out.println(); }); }}
