General

155 Min Stack

Use 2 stacks

//   |1|  
//   |3|      |1|
//   |2|      |2|
//  stack   minStack
public class MinStack{
    private Stack<Integer> stack;
    private Stack<Integer> minStack;
    public MinStack() {
        this.stack = new Stack<Integer>();
        this.minStack = new Stack<Integer>();
    }

    void push(int number) {
        stack.push(number);
        if (minStack.empty()) minStack.push(number);
        else{
            if (number <= minStack.peek()) minStack.push(number);
        }
    }

    int pop() {
        if (stack.peek().equals(minStack.peek())){ // compare using equals()
            minStack.pop();
        }
        return stack.pop();
    }

    int min() {
        return minStack.peek();
    }

Binary Tree node increasing order

1. curQueue 儲存當前node

2. nxtQueue 儲存下一層node

   public static List<List<Integer>>binaryTreeDepthOrder(BinaryTreeNode<Integer> tree){
    Queue<BinaryTreeNode<Integer>> curLevel = new LinkedList<>();
    List<List<Integer>> result = new ArrayList<>();
    curLevel.add(tree);
   
    while(!curLevel.isEmpty()){
        List<Integer> levelVal = new ArrayList<>();
        Queue<BinaryTreeNode<Integer>> nextLevel = new LinkedList<>();
   
        while(!curLevel.isEmpty()){
            BinaryTreeNode<Integer> node = curLevel.poll();
   
            // null check first, a null can still add into a Queue
            if (node != null){
                levelVal.add(node.data);
   
                nextLevel.add(node.left);
                nextLevel.add(node.right);
            }
        }
        if (!levelVal.isEmpty()) {
            result.add(levelVal);
        }
        curLevel = nextLevel;
    }
   
    return result;