LeetCode(力扣)：二叉树的前序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        //空树直接返回结果
        if(root == null){
            return result;
        }
        //新建栈并将根节点放入
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);

        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            result.add(node.val);
            //由于栈是先进后出，所以先将右节点放入
            if(node.right != null){
                stack.push(node.right);
            }
            if(node.left != null){
                stack.push(node.left);
            }
        }
        return result;
    }
}