class036 二叉树高频题目-上-不含树型dp
code1 102. 二叉树的层序遍历
// 二叉树的层序遍历
// 测试链接 : https://leetcode.cn/problems/binary-tree-level-order-traversal/
code1 普通bfs
code2 一次操作一层
package class036;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
// 二叉树的层序遍历
// 测试链接 : https://leetcode.cn/problems/binary-tree-level-order-traversal/
public class Code01_LevelOrderTraversal {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
}
// 提交时把方法名改为levelOrder,此方法为普通bfs,此题不推荐
public static List<List<Integer>> levelOrder1(TreeNode root) {
List<List<Integer>> ans = new ArrayList<>();
if (root != null) {
Queue<TreeNode> queue = new LinkedList<>();
HashMap<TreeNode, Integer> levels = new HashMap<>();
queue.add(root);
levels.put(root, 0);
while (!queue.isEmpty()) {
TreeNode cur = queue.poll();
int level = levels.get(cur);
if (ans.size() == level) {
ans.add(new ArrayList<>());
}
ans.get(level).add(cur.val);
if (cur.left != null) {
queue.add(cur.left);
levels.put(cur.left, level + 1);
}
if (cur.right != null) {
queue.add(cur.right);
levels.put(cur.right, level + 1);
}
}
}
return ans;
}
// 如果测试数据量变大了就修改这个值
public static int MAXN = 2001;
public static TreeNode[] queue = new TreeNode[MAXN];
public static int l, r;
// 提交时把方法名改为levelOrder,此方法为每次处理一层的优化bfs,此题推荐
public static List<List<Integer>> levelOrder2(TreeNode root) {
List<List<Integer>> ans = new ArrayList<>();
if (root != null) {
l = r = 0;
queue[r++] = root;
while (l < r) { // 队列里还有东西
int size = r - l;
ArrayList<Integer> list = new ArrayList<Integer>();
for (int i = 0; i < size; i++) {
TreeNode cur = queue[l++];
list.add(cur.val);
if (cur.left != null) {
queue[r++] = cur.left;
}
if (cur.right != null) {
queue[r++] = cur.right;
}
}
ans.add(list);
}
}
return ans;
}
}
code2 103. 二叉树的锯齿形层序遍历
// 二叉树的锯齿形层序遍历
// 测试链接 : https://leetcode.cn/problems/binary-tree-zigzag-level-order-traversal/
code 遍历
package class036;
import java.util.ArrayList;
import java.util.List;
// 二叉树的锯齿形层序遍历
// 测试链接 : https://leetcode.cn/problems/binary-tree-zigzag-level-order-traversal/
public class Code02_ZigzagLevelOrderTraversal {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
}
// 提交以下的方法
// 用每次处理一层的优化bfs就非常容易实现
// 如果测试数据量变大了就修改这个值
public static int MAXN = 2001;
public static TreeNode[] queue = new TreeNode[MAXN];
public static int l, r;
public static List<List<Integer>> zigzagLevelOrder(TreeNode root) {
List<List<Integer>> ans = new ArrayList<>();
if (root != null) {
l = r = 0;
queue[r++] = root;
// false 代表从左往右
// true 代表从右往左
boolean reverse = false;
while (l < r) {
int size = r - l;
ArrayList<Integer> list = new ArrayList<Integer>();
// reverse == false, 左 -> 右, l....r-1, 收集size个
// reverse == true, 右 -> 左, r-1....l, 收集size个
// 左 -> 右, i = i + 1
// 右 -> 左, i = i - 1
for (int i = reverse ? r - 1 : l, j = reverse ? -1 : 1, k = 0; k < size; i += j, k++) {
TreeNode cur = queue[i];
list.add(cur.val);
}
for (int i = 0; i < size; i++) {
TreeNode cur = queue[l++];
if (cur.left != null) {
queue[r++] = cur.left;
}
if (cur.right != null) {
queue[r++] = cur.right;
}
}
ans.add(list);
reverse = !reverse;
}
}
return ans;
}
}
code3 662. 二叉树最大宽度
// 二叉树的最大特殊宽度
// 测试链接 : https://leetcode.cn/problems/maximum-width-of-binary-tree/
package class036;
// 二叉树的最大特殊宽度
// 测试链接 : https://leetcode.cn/problems/maximum-width-of-binary-tree/
public class Code03_WidthOfBinaryTree {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
}
// 提交以下的方法
// 用每次处理一层的优化bfs就非常容易实现
// 如果测试数据量变大了就修改这个值
public static int MAXN = 3001;
public static TreeNode[] nq = new TreeNode[MAXN];
public static int[] iq = new int[MAXN];
public static int l, r;
public static int widthOfBinaryTree(TreeNode root) {
int ans = 1;
l = r = 0;
nq[r] = root;
iq[r++] = 1;
while (l < r) {
int size = r - l;
ans = Math.max(ans, iq[r - 1] - iq[l] + 1);
for (int i = 0; i < size; i++) {
TreeNode node = nq[l];
int id = iq[l++];
if (node.left != null) {
nq[r] = node.left;
iq[r++] = id * 2;
}
if (node.right != null) {
nq[r] = node.right;
iq[r++] = id * 2 + 1;
}
}
}
return ans;
}
}
code4 104. 二叉树的最大深度 111. 二叉树的最小深度
// 求二叉树的最大、最小深度
// 测试链接 : https://leetcode.cn/problems/maximum-depth-of-binary-tree/
// 测试链接 : https://leetcode.cn/problems/minimum-depth-of-binary-tree/
package class036;
// 求二叉树的最大、最小深度
public class Code04_DepthOfBinaryTree {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
}
// 测试链接 : https://leetcode.cn/problems/maximum-depth-of-binary-tree/
public static int maxDepth(TreeNode root) {
return root == null ? 0 : Math.max(maxDepth(root.left), maxDepth(root.right)) + 1;
}
// 测试链接 : https://leetcode.cn/problems/minimum-depth-of-binary-tree/
public int minDepth(TreeNode root) {
if (root == null) {
// 当前的树是空树
return 0;
}
if (root.left == null && root.right == null) {
// 当前root是叶节点
return 1;
}
int ldeep = Integer.MAX_VALUE;
int rdeep = Integer.MAX_VALUE;
if (root.left != null) {
ldeep = minDepth(root.left);
}
if (root.right != null) {
rdeep = minDepth(root.right);
}
return Math.min(ldeep, rdeep) + 1;
}
}
code5 297. 二叉树的序列化与反序列化
// 二叉树先序序列化和反序列化
// 测试链接 : https://leetcode.cn/problems/serialize-and-deserialize-binary-tree/
package class036;
// 二叉树先序序列化和反序列化
// 测试链接 : https://leetcode.cn/problems/serialize-and-deserialize-binary-tree/
public class Code05_PreorderSerializeAndDeserialize {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
public TreeNode(int v) {
val = v;
}
}
// 二叉树可以通过先序、后序或者按层遍历的方式序列化和反序列化
// 但是,二叉树无法通过中序遍历的方式实现序列化和反序列化
// 因为不同的两棵树,可能得到同样的中序序列,即便补了空位置也可能一样。
// 比如如下两棵树
// __2
// /
// 1
// 和
// 1__
// \
// 2
// 补足空位置的中序遍历结果都是{ null, 1, null, 2, null}
// 提交这个类
public class Codec {
public String serialize(TreeNode root) {
StringBuilder builder = new StringBuilder();
f(root, builder);
return builder.toString();
}
void f(TreeNode root, StringBuilder builder) {
if (root == null) {
builder.append("#,");
} else {
builder.append(root.val + ",");
f(root.left, builder);
f(root.right, builder);
}
}
public TreeNode deserialize(String data) {
String[] vals = data.split(",");
cnt = 0;
return g(vals);
}
// 当前数组消费到哪了
public static int cnt;
TreeNode g(String[] vals) {
String cur = vals[cnt++];
if (cur.equals("#")) {
return null;
} else {
TreeNode head = new TreeNode(Integer.valueOf(cur));
head.left = g(vals);
head.right = g(vals);
return head;
}
}
}
}
code6 105. 从前序与中序遍历序列构造二叉树
// 利用先序与中序遍历序列构造二叉树
// 测试链接 : https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
package class036;
// 二叉树按层序列化和反序列化
// 测试链接 : https://leetcode.cn/problems/serialize-and-deserialize-binary-tree/
public class Code06_LevelorderSerializeAndDeserialize {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
public TreeNode(int v) {
val = v;
}
}
// 提交这个类
// 按层序列化
public class Codec {
public static int MAXN = 10001;
public static TreeNode[] queue = new TreeNode[MAXN];
public static int l, r;
public String serialize(TreeNode root) {
StringBuilder builder = new StringBuilder();
if (root != null) {
builder.append(root.val + ",");
l = 0;
r = 0;
queue[r++] = root;
while (l < r) {
root = queue[l++];
if (root.left != null) {
builder.append(root.left.val + ",");
queue[r++] = root.left;
} else {
builder.append("#,");
}
if (root.right != null) {
builder.append(root.right.val + ",");
queue[r++] = root.right;
} else {
builder.append("#,");
}
}
}
return builder.toString();
}
public TreeNode deserialize(String data) {
if (data.equals("")) {
return null;
}
String[] nodes = data.split(",");
int index = 0;
TreeNode root = generate(nodes[index++]);
l = 0;
r = 0;
queue[r++] = root;
while (l < r) {
TreeNode cur = queue[l++];
cur.left = generate(nodes[index++]);
cur.right = generate(nodes[index++]);
if (cur.left != null) {
queue[r++] = cur.left;
}
if (cur.right != null) {
queue[r++] = cur.right;
}
}
return root;
}
private TreeNode generate(String val) {
return val.equals("#") ? null : new TreeNode(Integer.valueOf(val));
}
}
}
code7 958. 二叉树的完全性检验
// 验证完全二叉树
// 测试链接 : https://leetcode.cn/problems/check-completeness-of-a-binary-tree/
1)无左有右 flase
2)孩子不全开始 必须都是叶子结点,否则false
package class036;
import java.util.HashMap;
// 利用先序与中序遍历序列构造二叉树
// 测试链接 : https://leetcode.cn/problems/construct-binary-tree-from-preorder-and-inorder-traversal/
public class Code07_PreorderInorderBuildBinaryTree {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
public TreeNode(int v) {
val = v;
}
}
// 提交如下的方法
public static TreeNode buildTree(int[] pre, int[] in) {
if (pre == null || in == null || pre.length != in.length) {
return null;
}
HashMap<Integer, Integer> map = new HashMap<>();
for (int i = 0; i < in.length; i++) {
map.put(in[i], i);
}
return f(pre, 0, pre.length - 1, in, 0, in.length - 1, map);
}
public static TreeNode f(int[] pre, int l1, int r1, int[] in, int l2, int r2, HashMap<Integer, Integer> map) {
if (l1 > r1) {
return null;
}
TreeNode head = new TreeNode(pre[l1]);
if (l1 == r1) {
return head;
}
int k = map.get(pre[l1]);
// pre : l1(........)[.......r1]
// in : (l2......)k[........r2]
// (...)是左树对应,[...]是右树的对应
head.left = f(pre, l1 + 1, l1 + k - l2, in, l2, k - 1, map);
head.right = f(pre, l1 + k - l2 + 1, r1, in, k + 1, r2, map);
return head;
}
}
code8 222. 完全二叉树的节点个数
// 求完全二叉树的节点个数
// 测试链接 : https://leetcode.cn/problems/count-complete-tree-nodes/
package class036;
// 验证完全二叉树
// 测试链接 : https://leetcode.cn/problems/check-completeness-of-a-binary-tree/
public class Code08_CompletenessOfBinaryTree {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
}
// 提交以下的方法
// 如果测试数据量变大了就修改这个值
public static int MAXN = 101;
public static TreeNode[] queue = new TreeNode[MAXN];
public static int l, r;
public static boolean isCompleteTree(TreeNode h) {
if (h == null) {
return true;
}
l = r = 0;
queue[r++] = h;
// 是否遇到过左右两个孩子不双全的节点
boolean leaf = false;
while (l < r) {
h = queue[l++];
if ((h.left == null && h.right != null) || (leaf && (h.left != null || h.right != null))) {
return false;
}
if (h.left != null) {
queue[r++] = h.left;
}
if (h.right != null) {
queue[r++] = h.right;
}
if (h.left == null || h.right == null) {
leaf = true;
}
}
return true;
}
}
code9 222. 完全二叉树的节点个数
// 求完全二叉树的节点个数
// 测试链接 : https://leetcode.cn/problems/count-complete-tree-nodes/
package class036;
// 求完全二叉树的节点个数
// 测试链接 : https://leetcode.cn/problems/count-complete-tree-nodes/
public class Code09_CountCompleteTreeNodes {
// 不提交这个类
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
}
// 提交如下的方法
public static int countNodes(TreeNode head) {
if (head == null) {
return 0;
}
return f(head, 1, mostLeft(head, 1));
}
// cur : 当前来到的节点
// level : 当前来到的节点在第几层
// h : 整棵树的高度,不是cur这棵子树的高度
// 求 : cur这棵子树上有多少节点
public static int f(TreeNode cur, int level, int h) {
if (level == h) {
return 1;
}
if (mostLeft(cur.right, level + 1) == h) {
// cur右树上的最左节点,扎到了最深层
return (1 << (h - level)) + f(cur.right, level + 1, h);
} else {
// cur右树上的最左节点,没扎到最深层
return (1 << (h - level - 1)) + f(cur.left, level + 1, h);
}
}
// 当前节点是cur,并且它在level层
// 返回从cur开始不停往左,能扎到几层
public static int mostLeft(TreeNode cur, int level) {
while (cur != null) {
level++;
cur = cur.left;
}
return level - 1;
}
}