Binary Tree Preorder Traversal

地址：https://oj.leetcode.com/problems/binary-tree-preorder-traversal/

题意实现二叉树的前序遍历。利用栈实现。

public class Solution {       public List<Integer> preorderTraversal(TreeNode root) {    List<Integer > ans = new ArrayList<>();    TreeNode p = root;    Stack<TreeNode> st = new Stack<TreeNode>();    while(p!=null || st.size()!=0){    while(p!=null){    ans.add(p.val);    st.push(p);    p= p.left;    }    if(st.size()!=0){    p = st.peek();    st.pop();    p = p.right;    }    }    return ans;    }}