144.二叉树的前序遍历

题解

​ 每次迭代弹出栈顶元素，记录其根数值，按次序将其右孩子、左孩子压入栈中，

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
            LinkedList<TreeNode> stack = new LinkedList<>();
            LinkedList<Integer> output = new LinkedList<>();
            stack.add(root);
            if(root == null){
                return output;
            }
            while(!stack.isEmpty()){
                TreeNode node = stack.pollLast();
                output.add(node.val);
                if(node.right != null){
                    stack.add(node.right);
                }if(node.left != null){
                    stack.add(node.left);
                }
            }
            return output;
    }
}