前序遍历

迭代法

public class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<Integer>();

        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        if (root != null) stack.push(root);
        while (!stack.isEmpty()) {
            root = stack.pop();
            result.add(root.val);
            if (root.right != null) stack.push(root.right);
            if (root.left != null) stack.push(root.left);
        }

        return result;
    }
}

递归法

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    private List<Integer> result = new ArrayList<Integer>();

    public List<Integer> preorderTraversal(TreeNode root) {
        if (root != null) {
            result.add(root.val);
            preorderTraversal(root.left);
            preorderTraversal(root.right);
        }

        return result;
    }
}