cixuuz

class Solution {
	// O(n) O(n)
    public List<Integer> boundaryOfBinaryTree(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        
        if (root == null) return res;
        if (root.left == null && root.right == null) {
            res.add(root.val); 
            return res;
        }

cixuuz

class Solution {
  // O(n) O(n)
    public String optimalDivision(int[] nums) {
        StringBuilder sb = new StringBuilder();
        sb.append(nums[0]);
        for (int i = 1; i < nums.length; i++) {
            sb.append("/");
            if (i == 1 && nums.length>2) {
                sb.append("(");
            }

cixuuz

SELECT
    E1.id,
    E1.month,
    (IFNULL(E1.salary, 0) + IFNULL(E2.salary, 0) + IFNULL(E3.salary, 0)) AS Salary
FROM
    (SELECT
        id, MAX(month) AS month
    FROM
        Employee
    GROUP BY id

cixuuz

class Solution {
    public String tree2str(TreeNode t) {
        if (t == null) return "";
        String left = tree2str(t.left);
        String right = tree2str(t.right);
        
        StringBuilder sb = new StringBuilder();
        sb.append(t.val);
        if (right.equals("")) {
            if (!left.equals("")) sb.append("(" + left + ")");

cixuuz

class Solution {
    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
        if (t1 == null && t2 == null) return null;
        
        TreeNode node = new TreeNode(t1 == null? t2.val : t2 == null? t1.val : t1.val + t2.val);
        node.left = mergeTrees(t1 == null? null : t1.left, t2 == null? null : t2.left);
        node.right = mergeTrees(t1 == null? null : t1.right, t2 == null? null : t2.right);
        
        return node;
    }

cixuuz

class Solution {
	// O(n) O(1)
    public String solveEquation(String equation) {
        int x = 0, c = 0, num = -1, sign = 1; 
        boolean flag = false; // if at right of equal sign
        
        for (int i = 0; i < equation.length(); i++) {
            Character s = equation.charAt(i);
            if (Character.isDigit(s)) {
                if (num == -1) {

cixuuz

class Solution {
  // O(n) O(n)
    public int widthOfBinaryTree(TreeNode root) {
        if ( root == null ) return 0;
        
        Deque<TreeNode> queue = new LinkedList<TreeNode>();
        List<Integer> idx = new ArrayList<Integer>();
        
        queue.addLast(root);
        idx.add(0);

cixuuz

class Solution {
    private int res = 0;
    
    public int sumNumbers(TreeNode root) {
        if ( root == null ) return 0;
        dfs(root, 0);
        return res;
    }
    
    public void dfs(TreeNode node, int sum) {

cixuuz

class Solution {
    public int[][] imageSmoother(int[][] M) {
        if (M == null) return null;
        
        int rows = M.length;
        if (rows == 0) return new int[0][];
        int cols = M[0].length;

        int result[][] = new int[rows][cols];

cixuuz

public class Solution {
  // O(n) O(n)
    public void connect(TreeLinkNode root) {
        if ( root == null ) return;
        
        Deque<TreeLinkNode> deque = new LinkedList<TreeLinkNode>();
        deque.addLast(root);
        
        while ( !deque.isEmpty() ) {
            int n = deque.size();