二叉树的前、中、后序遍历（迭代版）

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// 前序遍历顺序：中-左-右，入栈顺序：中-右-左 class Solution {     public List<Integer> preorderTraversal(TreeNode root) {         List<Integer> result = new ArrayList<>();         if (root == null){             return result;         }         Stack<TreeNode> stack = new Stack<>();         stack.push(root);         while (!stack.isEmpty()){             TreeNode node = stack.pop();             result.add(node.val);             if (node.right != null){                 stack.push(node.right);             }             if (node.left != null){                 stack.push(node.left);             }         }         return result;     } }   // 中序遍历顺序: 左-中-右 入栈顺序： 左-右 class Solution {     public List<Integer> inorderTraversal(TreeNode root) {         List<Integer> result = new ArrayList<>();         if (root == null){             return result;         }         Stack<TreeNode> stack = new Stack<>();         TreeNode cur = root;         while (cur != null || !stack.isEmpty()){            if (cur != null){                stack.push(cur);                cur = cur.left;            }else{                cur = stack.pop();                result.add(cur.val);                cur = cur.right;            }         }         return result;     } }   // 后序遍历顺序 左-右-中 入栈顺序：中-左-右 出栈顺序：中-右-左， 最后翻转结果 class Solution {     public List<Integer> postorderTraversal(TreeNode root) {         List<Integer> result = new ArrayList<>();         if (root == null){             return result;         }         Stack<TreeNode> stack = new Stack<>();         stack.push(root);         while (!stack.isEmpty()){             TreeNode node = stack.pop();             result.add(node.val);             if (node.left != null){                 stack.push(node.left);             }             if (node.right != null){                 stack.push(node.right);             }         }         Collections.reverse(result);         return result;     } }