P26-二叉树遍历-层序-递归

//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 P26 {

    /*
    *           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);

        ArrayList list = new ArrayList();
        order(node1, 1, list);      //node1的下标记为1
        System.out.println(Arrays.toString(list.toArray()));
    }

    //层序-递归
    //1-2-3-4-5-6-7
    //i就是层级数
    public static void order(TreeNode root, int i, ArrayList list){

        if(root == null){
            return;
        }

        int length = list.size();
        if(length <= i){
            for(int j=0; j<= i-length; j++){
                list.add(length+j, null);
            }
        }


        list.set(i, root.val);
        order(root.left, 2*i, list);
        order(root.right, 2*i+1, list);
    }

    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;
        }
    }

}