【LeetCode-144】Binary Tree Preorder Traversal

public class BinaryTreePreorderTraversal {public class TreeNode {int val;TreeNode left;TreeNode right;TreeNode(int x) {val = x;}}public List<Integer> preorderTraversal(TreeNode root) {List<Integer> ls = new ArrayList<Integer>();if (root == null)return ls;//压入root节点Stack<TreeNode> st = new Stack<TreeNode>();st.push(root);//重点在这个循环while (!st.isEmpty()) {//取出栈顶节点，加入列表中TreeNode temp = st.pop();ls.add(temp.val);//右节点入栈if (temp.right != null)st.push(temp.right);//左节点入栈if (temp.left != null)st.push(temp.left);}return ls;}}