题解 | #KiKi学结构体和指针#

利用栈的方式进行二叉树的线序遍历

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型一维数组
     */

//     public int[] preorderTraversal (TreeNode root) {
//         // write code here
//         List<Integer> list = new ArrayList<>();
//         trans(list, root);
//         int[] res = new int[list.size()];
//         for(int i = 0; i < list.size(); i++){
//             res[i] = list.get(i);
//         }
//         return res;
//     }
//     public static void trans(List<Integer> queue, TreeNode root){
//         if(root != null){
//             queue.add(root.val);
//             trans(queue, root.left);
//             trans(queue, root.right);
//         }
//     }
    public int[] preorderTraversal (TreeNode root) {
        // write code here
        int[] a = {};
        if(root == null){
            return a;
        }
        ArrayList<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode node = stack.pop();
            list.add(node.val);
            if(node.right != null){
                stack.push(node.right);
            }
            if(node.left != null){
                stack.push(node.left);
            }
        }
        int[] res = new int[list.size()];
        for(int i = 0; i < list.size(); i++){
            res[i] = list.get(i);
        }
        return res;
    }
}