NC15 求二叉树的层序遍历

package NC;

import java.util.*;

/**
 * NC15 求二叉树的层序遍历
 *
 * 给定一个二叉树，返回该二叉树层序遍历的结果，（从左到右，一层一层地遍历）
 * 例如：
 * 给定的二叉树是{3,9,20,#,#,15,7},
 *
 * @author Tang
 * @date 2021/9/27
 */
public class LevelOrder {

    Map<Integer, ArrayList<Integer>> tempMap = new HashMap<>();


    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        postSearch(root, 0);

        ArrayList<ArrayList<Integer>> resultList = new ArrayList<>();
        for(int i = 0; i < tempMap.size(); i++) {
            ArrayList<Integer> list = tempMap.get(i);
            resultList.add(list);
        }

        return resultList;

    }

    /**
     * 后序遍历
     */
    private void postSearch(TreeNode node, int ceng) {
        if(node == null) {
            return;
        }

        postSearch(node.left, ceng+1);
        postSearch(node.right, ceng+1);


        //在特定层数的arrayList添加
        if(tempMap.get(ceng) == null) {
            ArrayList<Integer> arrayList = new ArrayList<>();
            tempMap.put(ceng, arrayList);
        }
        tempMap.get(ceng).add(node.val);

    }

    public static void main(String[] args) {

        TreeNode treeNode1 = new TreeNode(3);
        TreeNode treeNode2 = new TreeNode(9);
        TreeNode treeNode3 = new TreeNode(20);
        TreeNode treeNode4 = new TreeNode(15);
        TreeNode treeNode5 = new TreeNode(7);
        treeNode1.left = treeNode2;
        treeNode1.right = treeNode3;
        treeNode3.left = treeNode4;
        treeNode3.right = treeNode5;

        ArrayList<ArrayList<Integer>> arrayLists = new LevelOrder().levelOrder(treeNode1);




    }

}


class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

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