java 二叉树的后序遍历（非递归）

Posted 2021-05-08

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 
 /* 伪代码：
    1. 栈初始化
    2. 循环直到root为空且栈为空
        2.1 当root非空时循环
            2.1.1 将root连同标志flag=1入栈
            2.1.2 继续遍历root的左子树
        2.2 当栈非空 且 栈顶元素flag=2时循环
            2.2.1 栈顶元素弹出至root
            2.2.2 输出root.val
            2.2.3 root置为空
        2.3 如果栈非空，则
            2.2.1 栈顶元素的标志flag改为2
            2.2.2 准备遍历栈顶结点的右子树
  */
class BiTreePostOrder {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> stack = new ArrayDeque<>();
        Deque<Integer> flag = new ArrayDeque<>();
        while(root!=null || stack.size()!=0) {
            while(root!=null) {
                stack.addFirst(root);
                flag.addFirst(1);
                root = root.left;
            }
            while(stack.size()!=0 && flag.peekFirst()==2) {
                root = stack.removeFirst();
                flag.removeFirst();
                result.add(root.val);
                root = null;    //重要！否则程序陷入死循环，永远跳不出外层while循环
            }
            if(stack.size()!=0) {
                flag.removeFirst();
                flag.addFirst(2);
                root = stack.peekFirst().right;
            }
        }
        return result;
    }
}