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]
]

BFS 时间复杂度O(w),w是binary tree的最大宽度

/**
 * 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) {
        List<List<Integer>> res = new ArrayList<>();
        
        if(root == null)
            return res;
        
        Queue<TreeNode> queue = new LinkedList<>();
        
        queue.offer(root);
        
        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> list = new ArrayList<>();
            for(int i = 0; i < size; i++){
                TreeNode node = queue.poll();
                list.add(node.val);
                
                if(node.left != null){
                    queue.offer(node.left);
                }
                
                if(node.right != null){
                    queue.offer(node.right);
                }
                
            }
            
            res.add(list);
            
        }
        
        return res;
    }
}