二叉树的层次遍历

思路：

(1) 非递归思路： 队列

(2) 递归思路

package com.zhaochao.tree;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

/**
 * Created by zhaochao on 17/1/23.
 */
public class LeverTraversal {


    public ArrayList<List<Integer>> leverOrder(TreeNode root) {

        ArrayList<List<Integer>> result = new ArrayList<List<Integer>>();

        if(root == null) {
            return result;
        }

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);

        while(!queue.isEmpty()) {
            List<Integer> lever = new ArrayList<Integer>();
            int size = queue.size();
            for(int i = 0; i < size; i++) {
                TreeNode head = queue.poll();
                lever.add(head.val);
                if(head.left != null) {
                    queue.add(head.left);
                }
                if(head.right != null) {
                    queue.add(head.right);
                }
            }
            result.add(lever);
        }

        return result;
    }

    public static void main(String[] args) {

        TreeNode root = new TreeNode(0);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);

        root.left = node1;
        root.right = node2;
        node2.left = node3;

        LeverTraversal test = new LeverTraversal();
        ArrayList<List<Integer>> result = new ArrayList<List<Integer>>();
        result = test.leverOrder(root);
        for(int i = 0; i < result.size(); i++) {
            List<Integer> tmp = new ArrayList<Integer>();
            tmp = result.get(i);
            for(int j = 0; j < tmp.size(); j++) {
                System.out.print(tmp.get(j) + " ");
            }
            System.out.println();
        }
    }
}



递归思路：

package com.zhaochao.tree;

import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.List;

/**
 * Created by zhaochao on 17/1/23.
 */
public class LevelOrderRecursion {


    public ArrayList<ArrayList<Integer>> leverOrder(TreeNode root) {

        ArrayList<ArrayList<Integer>> result =new  ArrayList<ArrayList<Integer>>();
        if(root == null) {
            return result;
        }
        helper(result, 0, root);
        return result;
    }


    public void helper(ArrayList<ArrayList<Integer>> result, int level, TreeNode root) {
        if(root == null) {
            return;
        }

        ArrayList<Integer> curr;
        if(level >= result.size()) {
            curr = new ArrayList<Integer>();
            curr.add(root.val);
            result.add(curr);
        } else {
            curr = result.get(level);
            curr.add(root.val);
        }

        helper(result, level + 1, root.left);
        helper(result, level + 1, root.right);
    }


    public static void main(String[] args) {

        TreeNode root = new TreeNode(0);
        TreeNode node1 = new TreeNode(1);
        TreeNode node2 = new TreeNode(2);
        TreeNode node3 = new TreeNode(3);

        root.left = node1;
        root.right = node2;
        node2.left = node3;

        LevelOrderRecursion test = new LevelOrderRecursion();

        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        result = test.leverOrder(root);
        for(int i = 0; i < result.size(); i++) {
            List<Integer> tmp = new ArrayList<Integer>();
            tmp = result.get(i);
            for(int j = 0; j < tmp.size(); j++) {
                System.out.print(tmp.get(j) + " ");
            }
            System.out.println();
        }

    }
}