102. Binary Tree Level Order Traversal

最新推荐文章于 2022-02-20 23:20:19 发布

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最新推荐文章于 2022-02-20 23:20:19 发布

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Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree [3,9,20,null,null,15,7],

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
]

层次遍历，通过一个队列辅助实现，程序如下所示：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        ArrayDeque<TreeNode> que = new ArrayDeque<>();
        if (root != null){
            que.offer(root);
        }
        ArrayDeque<Integer> level = new ArrayDeque<>();
        level.offer(-1);
        List<Integer> list = new ArrayList<>();
        List<List<Integer>> llist = new ArrayList<>();
        while (!que.isEmpty()){
            TreeNode tmp = que.poll();
            list.add(tmp.val);
            int height = level.poll();
            if (tmp.left != null){
                que.offer(tmp.left);
                level.offer(height + 1);
            }
            if (tmp.right != null){
                que.offer(tmp.right);
                level.offer(height + 1);
            }
            if ((!level.isEmpty()&&height != level.peek())||que.isEmpty()){
                llist.add(list);
                list = new ArrayList<>();
            }
        }
        return llist;
    }
}