LeetCode --- 144. 二叉树的前序遍历

• LeetCode链接：144. 二叉树的前序遍历

解法1 - 递归

/**
 * 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) {
    	List<Integer> list = new ArrayList<>();
    	preorder(list,root);
    	return list;
    }
    private void preorder(List<Integer> list, TreeNode node) {
    	if (node == null) return; 
    	list.add(node.val);
    	preorder(list, node.left);
    	preorder(list, node.right);
	}
}

解法2 - 非递归

/**
 * 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) {
    	List<Integer> list = new ArrayList<>();
        if (root == null) return list;
        Stack<TreeNode>stack = new Stack<>();
        stack.push(root);
    	while (root != null || !stack.isEmpty()) {
			if (root != null) {
				list.add(root.val);
				stack.push(root);
				root = root.left;
			} else {
				TreeNode node = stack.pop();
				root = node.right;
			}
		}
        return list;
	}
}