数据结构 二叉树OJ题

结合之前所学的二叉树知识，我们来刷一些OJ题巩固一下：

1. 检查两颗二叉树是否相同

OJ链接

代码示例：

class Solution {public boolean isSameTree(TreeNode p,TreeNode q) {if ((p == null && q != null) || (p != null && q == null)) {return false;}if (p == null && q == null) {return true;}if (p.val != q.val) {return false;}return isSameTree(p.left,q.left) && isSameTree(p.right,q.right);}
}

2. 判断树是否为另一个树的子树

OJ链接

代码示例：

class Solution {public boolean isSameTree(TreeNode p,TreeNode q) {if ((p == null && q != null) || (p != null && q == null)) {return false;}if (p == null && q == null) {return true;}if (p.val != q.val) {return false;}return isSameTree(p.left,q.left) && isSameTree(p.right,q.right);}public boolean isSubtree(TreeNode root,TreeNode subRoot) {if (root == null || subRoot == null) {return false;}if (isSameTree(root,subRoot)) {return true;}if (isSubtree(root.left,subRoot)) {return true;}if (isSubtree(root.right,subRoot)) {return true;}return false;}
}

3. 翻转二叉树

OJ链接

代码示例：

class Solution {public TreeNode invertTree(TreeNode root) {if (root == null) {return null;}if (root.left == null && root.right == null) {return root;}TreeNode temp = root.left;root.left = root.right;root.right = temp;invertTree(root.left);invertTree(root.right);return root;}
}

4. 平衡二叉树

OJ链接

代码示例：

class Solution {public boolean isBalanced(TreeNode root) {if (root == null) {return true;}int leftH = getHeight(root.left);int rightH = getHeight(root.right);if (Math.abs(leftH-rightH) > 1) {return false;}return isBalanced(root.left) && isBalanced(root.right);}public int getHeight(TreeNode root) {if (root == null) {return 0;}int leftNode = getHeight(root.left);int rightNode = getHeight(root.right);return leftNode > rightNode ? leftNode + 1 : rightNode + 1;}
}

5. 对称二叉树

OJ链接

代码示例：

class Solution {public boolean isSymmetric(TreeNode root) {if (root == null) {return true;}return isSymmetricChild(root.left,root.right);}private boolean isSymmetricChild(TreeNode leftTree, TreeNode rightTree) {if (leftTree == null && rightTree == null) {return true;}if ((leftTree == null && rightTree != null) || (leftTree != null && rightTree == null)) {return false;}if (leftTree.val != rightTree.val) {return false;}return isSymmetricChild(leftTree.left,rightTree.right) && isSymmetricChild(leftTree.right,rightTree.left);}
}

6. 二叉树遍历

[OJ链接]:

代码示例：

import java.util.Scanner;public class Main {public static int i = 0;static class TreeNode {char val;TreeNode left;TreeNode right;public TreeNode(char val) {this.val = val;}}public static void main(String[] args) {Scanner in = new Scanner(System.in);String s1 = in.nextLine();TreeNode root = createTree(s1);isOrder(root);}public static TreeNode createTree(String s) {if (s.charAt(i) != '#') {TreeNode root = new TreeNode(s.charAt(i));i++;root.left = createTree(s);root.right = createTree(s);return root;}else {i++;}return null;}public static void isOrder(TreeNode root) {if (root == null) {return;}isOrder(root.left);System.out.print(root.val + " ");isOrder(root.right);}
}

7. 二叉树层序遍历

OJ链接

代码示例：

class Solution {public List<List<Integer>> levelOrder(TreeNode root) {List<List<Integer>> ret1 = new LinkedList<>();Queue<TreeNode> queue = new LinkedList<>();if (root == null) {return ret1;}queue.offer(root);while(!queue.isEmpty()) {List<Integer> list = new LinkedList<>();int size = queue.size();while(size != 0) {TreeNode cur = queue.poll();size--;System.out.print(cur.val + " ");if (cur.left != null) {queue.offer(cur.left);}if (cur.right != null) {queue.offer(cur.right);}list.add(cur.val);}ret1.add(list);}return ret1;}
}

8. 最近公共祖先

OJ链接

代码示例：

class Solution {public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {if (root == null) {return null;}if (root == p || root == q) {return root;}TreeNode leftTree = lowestCommonAncestor(root.left,p,q);TreeNode rightTree = lowestCommonAncestor(root.right,p,q);if (leftTree != null && rightTree != null) {return root;}else if(leftTree != null) {return leftTree;}else {return rightTree;}}
}

9. 二叉树创建字符串

[OJ链接]:

代码示例：

class Solution {public String tree2str(TreeNode root) {StringBuilder stringBuilder = new StringBuilder();tree2strChild(root,stringBuilder);return stringBuilder.toString();}private void tree2strChild(TreeNode root, StringBuilder stringBuilder) {if (root == null) {return;}stringBuilder.append(root.val + "");if (root.left != null) {stringBuilder.append("(");tree2strChild(root.left,stringBuilder);stringBuilder.append(")");}else {if (root.right == null) {return;}else{stringBuilder.append("()");}}if (root.right != null) {stringBuilder.append("(");tree2strChild(root.right,stringBuilder);stringBuilder.append(")");}else {return;}}
}   

10. 非递归方式实现前序遍历

[OJ链接]:

代码示例：

class Solution {public List<Integer> preorderTraversal(TreeNode root) {List<Integer> list = new LinkedList<>();if (root == null) {return list;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while(cur != null || !stack.isEmpty()) {while(cur != null) {stack.push(cur);list.add(cur.val);cur = cur.left;}TreeNode top = stack.pop();cur = top.right;}return list;}}

11. 非递归方式实现中序遍历

[OJ链接]:

代码示例：

class Solution {public List<Integer> inorderTraversal(TreeNode root) {List<Integer> list = new LinkedList<>();if (root == null) {return list;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while(cur != null || !stack.empty()) {while(cur != null) {stack.push(cur);cur = cur.left;}TreeNode top = stack.pop();list.add(top.val);cur = top.right;}return list;}
}

12. 非递归方式实现后序遍历

[OJ链接]:

代码示例：

class Solution {public List<Integer> postorderTraversal(TreeNode root) {List<Integer> list = new LinkedList<>();if (root == null) {return list;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;TreeNode prev = null;while(cur != null || !stack.isEmpty()) {while(cur != null) {stack.push(cur);cur = cur.left;}TreeNode top = stack.peek();if (top.right == null || top.right == prev) {list.add(top.val);stack.pop();prev = top; // 记录下最新被打印的节点}else {cur = top.right;}}return list;}
}