cixuuz

public class Solution {
    public List<String> binaryTreePaths(TreeNode root) {
        List<String> res = new ArrayList<String>();
        if (root != null) dfs(root, "", res);
        return res;
    }
    private void dfs(TreeNode root, String path, List<String> res) {
        if (root.left == null && root.right == null) res.add(path + root.val);
        if (root.left != null) dfs(root.left, path+root.val+"->", res);
        if (root.right != null) dfs(root.right, path+root.val+"->", res);

cixuuz

public class Solution {
    private TreeNode firstNode = null;
    private TreeNode secondNode = null;
    private TreeNode preNode = new TreeNode(Integer.MIN_VALUE);
        
    public void recoverTree(TreeNode root) {
        traverse(root);
        
        Integer tmp = firstNode.val;
        firstNode.val = secondNode.val;

cixuuz

public class Solution {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        return dfs(preorder, inorder, 0, 0, preorder.length);
    }
    
    private TreeNode dfs(int[] preorder, int[] inorder, int preStart, int inStart, int inEnd) {
        if (inStart > inEnd || preStart >= preorder.length) return null;

        TreeNode node = new TreeNode(preorder[preStart]);
        

cixuuz

public class Codec {
    private static final String SEP = ",";
    private static final String NULL = "x";
    
    // Encodes a tree to a single string.
    public String serialize(TreeNode root) {
        StringBuilder sb = new StringBuilder();
        buildString(root, sb);
        return sb.toString();
    }

cixuuz

public class Solution {
    public boolean hasPathSum(TreeNode root, int sum) {
        if (root == null) return false;
        if (root.left == null && root.right == null) return sum == root.val;
        
        sum -= root.val;
        boolean l = hasPathSum(root.left, sum);
        boolean r = hasPathSum(root.right, sum);
        sum += root.val;
        return l || r;

cixuuz

public class Solution {
    public int countNodes(TreeNode root) {
        if (root == null)
            return 0;
        int lh = height(root.left);
        int rh = height(root.right);
        if (lh == rh) return (1 << lh) + countNodes(root.right);
        else return (1 << rh) + countNodes(root.left);
    }
    

cixuuz

public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        
        if (root == null) return res;
        
        q.offer(root);
        while (!q.isEmpty()) {
            Integer levelLength = q.size();

cixuuz

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        Map<Integer, Integer> visited = new HashMap<>();
        
        for (int i=0; i<nums.length; i++) {
            if (visited.containsKey(nums[i])) {
                result[0] = visited.get(nums[i]);
                result[1] = i;
                return result;

cixuuz

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

cixuuz

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {