102. Binary Tree Level Order Traversal

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],

    3
   / \
  9  20
    /  \
   15   7

return its level order traversal as:

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

层序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> re = new LinkedList<List<Integer>>();
        if (root == null) {
            return re;
        }
        ArrayDeque<TreeNode> queue = new ArrayDeque<TreeNode>();
        TreeNode tmp;
        int size = 0;
        queue.offer(root);
        while (!queue.isEmpty()) {
            List<Integer> l = new LinkedList<Integer>();
            size = queue.size();
            for (int i = 0; i < size; i++) {
                tmp = queue.poll();
                if (tmp.left != null) {
                    queue.offer(tmp.left);
                }
                if (tmp.right != null) {
                    queue.offer(tmp.right);
                }
                l.add(tmp.val);
            }
            re.add(l);
        }
        return re;
    }
}