102. 二叉树的层次遍历

递推

package leetcode;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

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

	TreeNode(int x) {
		val = x;
	}
}

class Solution {
	public List<List<Integer>> levelOrder(TreeNode root) {
		List<List<Integer>> ls = new ArrayList<List<Integer>>();
		Queue<TreeNode> queue = new LinkedList<TreeNode>();

		if (root == null)
			return ls;

		queue.add(root);
		while (!queue.isEmpty()) {				                            	 // 结合数学运算
			int len = queue.size();
			List<Integer> l = new ArrayList<Integer>();
			for (int i = 0; i < len; i++) {                                                        //len用于记录queue.size() 不会被for 里面的代码所影响  这道题 很妙
				TreeNode T = queue.poll();
				
				if (T.left != null)
					queue.add(T.left);
				if (T.right != null)
					queue.add(T.right);

				l.add(T.val);
			}
			ls.add(l);
                         
		}
		return ls;
	}
}

解法二

递归

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> list = new ArrayList<>();
        levelOrder(root, list, 0);
        return list;
    }
    
    public void levelOrder(TreeNode root, List<List<Integer>> list, int level){
        if(root != null){
            if(level > list.size() - 1){
                List<Integer> nodes = new ArrayList<>();
                list.add(nodes);
            }
            list.get(level).add(root.val);
            levelOrder(root.left, list, level + 1);
            levelOrder(root.right, list, level + 1);
        }
    }
}