用迭代法实现二叉树的前、中、后序遍历

前序：
 public List<Integer> inorderTraversal(TreeNode root) {
     List<Integer> res=new ArrayList<Integer>();
     if (root==null) {
     return res;
    }
                     
                       Stack<TreeNode> stack=new Stack<>();
                     stack.push(root);
                       while (!stack.isEmpty()) {
      TreeNode node=stack.pop();
      res.add(node.val);
      if (node.right!=null) {
       stack.push(node.right);
      }
      if (node.left!=null) {
       stack.push(node.left);
      }
     }
                       return res;
       }

中序：
/**
 * 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) {
   List<Integer> res=new ArrayList<Integer>();
                       Stack<TreeNode> stack=new Stack<>();
                       while (root!=null||stack.size()>0) {
      if (root!=null) {
       stack.add(root);
       root=root.left;
      }
      else {
       TreeNode temp=stack.pop();
       res.add(temp.val);
       root=temp.right;
      }
     }
                       return res;
    }
}

后序：把前序变一下，然后逆序
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
  List<Integer> res=new ArrayList<>();
              if (root==null) {
    return res;
   }
              Stack<TreeNode> stack=new Stack<>();
              while (root!=null||!stack.isEmpty()) {
    if (root!=null) {
     res.add(root.val);
     stack.push(root);
     root=root.right;
    }
    else {
     TreeNode temp=stack.pop();
     root=temp.left;
    }
   }
              Collections.reverse(res);
              return res;
    }
}