LC144. Binary Tree Preorder Traversal
Input: [1,null,2,3]
1

2
/
3

Output: [1,2,3]

  public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        
        while (!stack.isEmpty()){
            TreeNode node = stack.pop();
            list.add(node.val);
            if (node.right != null) stack.push(node.right);
            if (node.left != null) stack.push(node.left);
        }
        
        return list;
    }

LC94. Binary Tree Inorder Traversal
Input: [1,null,2,3]
1

2
/
3

Output: [1,3,2]

public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        
        while (curr != null || !stack.isEmpty()){
            while (curr != null){
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            list.add(curr.val);
            curr = curr.right;
        }
        return list;
    }

LC107. Binary Tree Level Order Traversal II
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]

 public List<List<Integer>> levelOrderBottom(TreeNode root) {
        List<List<Integer>> list = new ArrayList<List<Integer>>();
        if (root == null) return list;
        Queue<TreeNode> q = new LinkedList<TreeNode>();
        q.offer(root); // instead of using q.add(belongs to collection)
        while(!q.isEmpty()){
            List<Integer> list1 = new ArrayList<Integer>();
            int size = q.size();
            for (int i = 0; i < size; i++){
                if (q.peek().left != null)
                    q.offer(q.peek().left);
                if (q.peek().right != null)
                    q.offer(q.peek().right);
                list1.add(q.poll().val);// instead of remove to get element and remove.
            }
            list.add(0,list1); //move list1 to the  
        }
        return list;
    }

LC98. Validate Binary Search Tree
5
/
1 4
/
3 6
Output: false
Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value
is 5 but its right child's value is 4.

class ResultType {
    int maxValue, minValue;
    boolean isBST;
    public ResultType(boolean isBST, int minValue, int maxValue) {
        this.maxValue = maxValue;
        this.minValue = minValue;
        this.isBST = isBST;
    }
}
public class Solution {
    public boolean isValidBST(TreeNode root) {
        return helper(root).isBST; 
    }
    
    private ResultType helper(TreeNode root){
        if (root == null) return new ResultType(true, Integer.MAX_VALUE, Integer.MIN_VALUE);
        
        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        
        if (!left.isBST || !right.isBST)
            return new ResultType(false, 0, 0);
        if (root.left!= null && left.maxValue >= root.val || root.right != null && right.minValue <= root.val) 
            return new ResultType(false, 0, 0);
        return new ResultType(true, Math.min(root.val, left.minValue), Math.max(root.val, right.maxValue));
        
    }
}

LC104. Maximum Depth of Binary Tree
Given binary tree [3,9,20,null,null,15,7],

3

/
9 20
/
15 7
return its depth = 3.

 public int maxDepth(TreeNode root) {
        if (root == null) return 0;
        
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        
        return Math.max(left, right) + 1;
    }

LC110. Balanced Binary Tree

class ResultType {
    int maxDepth;
    boolean isBalanced;
    public ResultType (int maxDepth, boolean isBalanced){
        this.maxDepth = maxDepth;
        this.isBalanced = isBalanced; //返回深度和是否是平衡
    }
}

public class Solution {
    public boolean isBalanced(TreeNode root) {
        return helper(root).isBalanced;
    }
    public ResultType helper(TreeNode root){
        if (root == null) return new ResultType(0, true); // 新类需要加new;
        
        ResultType left = helper(root.left);
        ResultType right = helper(root.right);
        
        if (left.isBalanced == false || right.isBalanced == false || Math.abs(left.maxDepth-right.maxDepth) > 1){
            return new ResultType(-1, false);
        }
        
        return new ResultType(Math.max(left.maxDepth, right.maxDepth) + 1, true);
    }
}