题目一：不分行从上到下打印二叉树
从上往下打印出二叉树的每个节点，同层节点从左至右打印。
练习地址
https://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701
https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-lcof/
参考答案
import java.util.ArrayList;
import java.util.LinkedList;
/**
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
ArrayList<Integer> result = new ArrayList<>();
if (root == null) {
return result;
}
LinkedList<TreeNode> nodeList = new LinkedList<>();
nodeList.add(root);
while (!nodeList.isEmpty()) {
TreeNode node = nodeList.removeFirst();
result.add(node.val);
if (node.left != null) {
nodeList.add(node.left);
}
if (node.right != null) {
nodeList.add(node.right);
}
}
return result;
}
}
复杂度分析
- 时间复杂度：O(n)。
- 空间复杂度：O(n)。
题目二：分行从上到下打印二叉树
从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。
练习地址
https://www.nowcoder.com/practice/445c44d982d04483b04a54f298796288
https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/
参考答案
import java.util.ArrayList;
import java.util.LinkedList;
/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
ArrayList<ArrayList<Integer>> Print(TreeNode pRoot) {
ArrayList<ArrayList<Integer>> result = new ArrayList<>();
if (pRoot == null) {
return result;
}
LinkedList<TreeNode> nodeList = new LinkedList<>();
nodeList.add(pRoot);
// 下一层的节点数
int nextLevel = 0;
// 在当前层中还没有打印的节点数
int toBePrinted = 1;
ArrayList<Integer> levelList = new ArrayList<>();
while (!nodeList.isEmpty()) {
TreeNode node = nodeList.removeFirst();
levelList.add(node.val);
if (node.left != null) {
nodeList.add(node.left);
nextLevel++;
}
if (node.right != null) {
nodeList.add(node.right);
nextLevel++;
}
if (--toBePrinted == 0) {
result.add(levelList);
toBePrinted = nextLevel;
nextLevel = 0;
levelList = new ArrayList<>();
}
}
return result;
}
}
复杂度分析
- 时间复杂度：O(n)。
- 空间复杂度：O(n)。
题目三：之字形打印二叉树
请实现一个函数按照之字形打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右至左的顺序打印，第三行按照从左到右的顺序打印，其他行以此类推。
练习地址
https://www.nowcoder.com/practice/91b69814117f4e8097390d107d2efbe0
https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/
参考答案
import java.util.ArrayList;
import java.util.Stack;
/*
public class TreeNode {
int val = 0;
TreeNode left = null;
TreeNode right = null;
public TreeNode(int val) {
this.val = val;
}
}
*/
public class Solution {
public ArrayList<ArrayList<Integer> > Print(TreeNode pRoot) {
ArrayList<ArrayList<Integer>> result = new ArrayList<>();
if (pRoot == null) {
return result;
}
Stack<TreeNode>[] stacks = new Stack[2];
stacks[0] = new Stack<>();
stacks[1] = new Stack<>();
int current = 0;
stacks[0].push(pRoot);
ArrayList<Integer> levelList = new ArrayList<>();
while (!stacks[0].isEmpty() || !stacks[1].isEmpty()) {
TreeNode node = stacks[current].pop();
levelList.add(node.val);
if (current == 0) {
if (node.left != null) {
stacks[1].push(node.left);
}
if (node.right != null) {
stacks[1].push(node.right);
}
} else {
if (node.right != null) {
stacks[0].push(node.right);
}
if (node.left != null) {
stacks[0].push(node.left);
}
}
if (stacks[current].isEmpty()) {
result.add(levelList);
levelList = new ArrayList<>();
current = 1 - current;
}
}
return result;
}
}
复杂度分析
- 时间复杂度：O(n)。
- 空间复杂度：O(n)。
