Binary Tree Level Order Traversal

LeetCode LintCode

注意：Level Order Traversal的时候基本都用BFS
这道题是二叉树上的宽度优先搜索

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Level order a list of lists of integer
     */
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        // write your code here
        ArrayList<ArrayList<Integer>> results = new ArrayList<>();
        if (root == null){
            return results;
        }
        Queue<TreeNode>  queue = new LinkedList<TreeNode>();
        queue.offer(root);
        while (!queue.isEmpty()){
            ArrayList<Integer> level = new ArrayList<>();
            int size = queue.size();
            for (int i = 0; i < size; i++){
                TreeNode head = queue.poll();
                level.add(head.val);
                if (head.left != null){
                    queue.offer(head.left);
                }
                if (head.right != null){
                    queue.offer(head.right);
                }
            }
            results.add(level);
        }
        return results;
    }
}

这个视频讲得很好：
https://www.youtube.com/watch?v=86g8jAQug04