树的中序遍历（非递归）

public class Solution {
public List<Integer> inorderTraversal(TreeNode root) {
List<Integer> list = new ArrayList<Integer>();
if (root == null)
return list;
Stack<TreeNode> stack = new Stack<TreeNode>();
while (root != null || !stack.empty()) {
while (root != null) {
stack.push(root);
root = root.left;
}
if (!stack.empty()) {
TreeNode node = stack.pop();
list.add(node.val);
root = node.right;
}
}

return list;
}
}