二叉树的中序遍历——迭代法

/**
 * 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> inorderTraversal(TreeNode root) {
        ArrayDeque<TreeNode> stack = new ArrayDeque<>();
        List<Integer> list = new LinkedList<>();
        if (root == null) return list;
        stack.push(root);
        while (stack.peek().left != null) {
            stack.push(stack.peek().left);
        }
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            list.add(node.val);
            if (node.right != null) {
                stack.push(node.right);
                while (stack.peek().left != null) {
                    stack.push(stack.peek().left);
                }
            }
            
        }
        return list;
    }
}