Enter two integer sequences. The first sequence represents the push order of the stack. Please judge whether the second sequence is the pop-up order of the stack. Assume that all the numbers pushed into the stack are not equal. For example, sequence 1,2,3,4,5 is the pressing sequence of a stack, sequence 4,5,3,2,1 is a pop-up sequence corresponding to the pressing sequence, but 4,3,5,1,2 cannot be the pop-up sequence of the pressing sequence. (Note: the two sequences are the same length)

import java.util.*;

public class Solution {
    public boolean IsPopOrder(int [] pushA,int [] popA) {
      if(pushA==null || popA==null) return false;
        Stack<Integer> stack = new Stack<>();
        int k=0;
        for (int i = 0; i < pushA.length; i++) {
            stack.push(pushA[i]);
            while(!stack.empty() && stack.peek() == popA[k]){
                stack.pop();
                k++;
            }

        }
        return stack.empty();
    }
}

Each node of the binary tree is printed from top to bottom, and the nodes of the same layer are printed from left to right.

public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> arr = new ArrayList<>();
        if(root == null) return arr;
        Deque<TreeNode> que = new LinkedList<>();
        que.add(root);
        while(!que.isEmpty()){
            TreeNode a = que.pop();
            arr .add(a.val);
            if(a.left!=null) que.add(a.left);
            if(a.right!=null) que.add(a.right);
        }
        return arr;
    }
}

Input an integer array to determine whether the array is the result of the subsequent traversal of a binary search tree. If Yes, output Yes; otherwise, output No. Suppose that any two numbers of the input array are different from each other.

public class Solution {
    public boolean VerifySquenceOfBST(int [] sequence) {
        if(sequence.length ==0) return false;
        return IsTreeBST(sequence, 0, sequence.length-1);
    }

    private boolean IsTreeBST(int[] sequence, int start, int end) {
        if(start>=end) return true;
        int i;
        for (i=start; i < end; i++) {
            if(sequence[i] >sequence[end])
                break;
        }
        for (int j = i; j < end; j++) {
            if(sequence[j] < sequence[end])
                return false;
        }
        return IsTreeBST(sequence,start,i-1) && IsTreeBST(sequence,i,end-1);
    }
}