二叉树前序遍历

非递归求解前序二叉树

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList
     */
    public ArrayList<Integer> preorderTraversal (TreeNode root) {
        // write code here
        ArrayList<Integer> res = new ArrayList<>();

        if(root == null) return res;

        Stack<TreeNode> st = new Stack<>();
        st.push(root);

        while(!st.isEmpty()){
            TreeNode node = st.pop();
            res.add(node.val);
            if(node.right != null){st.push(node.right);}
            if(node.left != null){st.push(node.left);}
        }

        return res;
    }
}