二叉树常见操作

public class  {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode(int x) {
        val = x;
    }

    
    public String toString() {
        return "" + val;
    }
}


public List<TreeNode> getPreOrderIteratively(TreeNode p) {
    return getPreOrderIteratively(p, true);
}

//对于非叶子节点，如果有一个子树位空会把它打印出来
private List<TreeNode> getPreOrderIteratively(TreeNode p, boolean printNull) {
    Stack<TreeNode> stack = new Stack<>();
    List<TreeNode> listP = new ArrayList<>();
    while (p != null || !stack.isEmpty()) {
        while (p != null) {
            listP.add(p);
            stack.push(p);
            if (printNull && !isLeaf(p) && p.left == null) {
                listP.add(null);
            }
            p = p.left;
        }
        if (!stack.isEmpty()) {
            p = stack.pop();
            if (printNull && !isLeaf(p) && p.right == null) {
                listP.add(null);
            }
            p = p.right;
        }
    }
    return listP;
}

private boolean isLeaf(TreeNode parent) {
    if (parent.left == null && parent.right == null) {
        return true;
    }
    return false;
}

private boolean isSameTreeNode(TreeNode p1, TreeNode p2) {
    if (p1 == null && p2 == null) {
        return true;
    }
    if (p1 == null || p2 == null) {
        return false;
    }
    return p1.val == p2.val;
}


//递归的层次遍历
public List<List<Integer>> levelOrderRecursively (TreeNode root) {
    List<List<Integer>> result = new ArrayList<>();
    result = levelOrderInternal(root, result,0);
    return result;
}

private List<List<Integer>> levelOrderInternal(TreeNode node, List<List<Integer>> result, int level){
    if(node == null) return result;
    if(result.size() <= level){
        result.add(new ArrayList<>());
    }
    result.get(level).add(node.val);
    levelOrderInternal(node.left,result, level+1);
    levelOrderInternal(node.right,result,level+1);
    return result;
}

//非递归的层次遍历
public List<List<Integer>> levelOrderIteratively(TreeNode root) {
    List<List<Integer>> ret = new ArrayList<>();
    if(root ==null){
        return ret;
    }

    Queue<TreeNode> queue = new LinkedList<>();
    List<Integer> list = new ArrayList<>();

    TreeNode levelStart = root ;
    queue.offer(root);

    while(!queue.isEmpty()){
        TreeNode node = queue.poll();
        if(node == levelStart){
            list = new ArrayList<>();
            ret.add(list);
            levelStart = null ;
        }
        list.add(node.val);

        if(node.left !=null){
            queue.offer(node.left);
            if(levelStart == null){
                levelStart = node.left;
            }      
        }
        if(node.right !=null){
            queue.offer(node.right);
            if(levelStart == null){
                levelStart = node.right ; 
            } 
        }
    }
    return ret ;
}