面试题-算法：二叉树的前序遍历

public class Tree {

    //定义一颗树
public static class TreeNode{
int val;        //左孩子
private TreeNode left;        //右孩子
private TreeNode right;
        TreeNode(int x){
            val = x;
        }
    }

public static List<Integer> preOrder(TreeNode root){        //结果
List<Integer> list = new LinkedList<>();        //栈
Stack<TreeNode> stack = new Stack<>();
        if(root ==null){
            return list;
        }

        stack.push(root);
while(!stack.isEmpty()){            //栈顶元素出栈
TreeNode treeNode = stack.pop();
list.add(treeNode.val);            //左孩子是否存在
if(treeNode.right!=null){
                stack.push(treeNode.right);
}            //右孩子是否存在
if(treeNode.left!=null){
                stack.push(treeNode.left);
            }
        }
        return list;
    }

    public static void main(String[] args) {
        //初始化二叉树
        TreeNode root = new TreeNode(1);
        root.left = new TreeNode(2);
        root.right = new TreeNode(3);
        root.left.left = new TreeNode(4);
        root.left.right = new TreeNode(5);
        
        //例如:上述初始化的二叉树，前序遍历输出1,2,4,5,3
        List<Integer> integers = preOrder(root);
        System.out.println(integers);
    }

}