P29-二叉树遍历-后序-迭代

//二叉树遍历
/*
 * 前序遍历：根左右
 * 中序遍历：左根右
 * 后序遍历：左右根
 * 层序遍历：从上往下、从左往右
 *
 * 递归遍历：使用递归方法遍历
 * 迭代遍历：使用迭代方法实现递归函数，与递归等价
 * morris遍历
 * */
public class P29 {

    /*
     *           1
     *         /   \
     *       2       3
     *     /   \
     *   4       5
     *         /   \
     *       6       7
     * */
    public static void main(String[] args) {
        TreeNode node7 = new TreeNode(7, null, null);
        TreeNode node6 = new TreeNode(6, null, null);
        TreeNode node5 = new TreeNode(5, node6, node7);
        TreeNode node4 = new TreeNode(4, null, null);
        TreeNode node3 = new TreeNode(3, null, null);
        TreeNode node2 = new TreeNode(2, node4, node5);
        TreeNode node1 = new TreeNode(1, node2, node3);

        iter(node1);
    }

    //后序-迭代
    //4-6-7-5-2-3-1
    public static void iter(TreeNode root){

        if(root != null){
            Stack<TreeNode> stack = new Stack<>();
            TreeNode prev = null;
            while(!stack.isEmpty() || root != null){
                while(root != null){
                    stack.push(root);       //不停让左节点进栈
                    root = root.left;
                }

                root = stack.pop();

                if(root.right == null || root.right == prev){
                    System.out.println(root.val);
                    prev = root;
                    root = null;        //防止重复进栈出栈死循环
                }else{      //上面的while已经是确定了没有左节点了，这个else是说还有右节点，要把节点重新进栈，但不要输出，因为要先把右孩子的所有先输出了
                    stack.push(root);
                    root = root.right;
                }
            }
        }
    }

    static class TreeNode{
        int val;
        TreeNode left;
        TreeNode right;
        int deep;

        TreeNode(){}

        TreeNode(int val){
            this.val = val;
        }

        TreeNode(int val, TreeNode left, TreeNode right){
            this.val = val;
            this.left = left;
            this.right = right;
        }
    }

}