LeetCode(102)——Binary Tree Level Order Traversal

題目：

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]
]

 

層序遍歷，把每層節點的數存下來。

AC:

public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> resultList = new ArrayList<>();
        
        List<TreeNode> temp = new LinkedList<>();
        
        if (root != null) {
            temp.add(root);
        }
        
        
        while (!temp.isEmpty()) {
            List<Integer> level = new ArrayList<>();

            int size = temp.size();
            for (int i = 0; i < size; i++) {
                TreeNode tmpNode = temp.get(0);
                
                level.add(tmpNode.val);
                
                if (tmpNode.left != null) {
                    temp.add(tmpNode.left);
                }
                
                if (tmpNode.right != null) {
                    temp.add(tmpNode.right);
                }
                
                temp.remove(0);
            }
            
            resultList.add(level);
        }
        
        return resultList;
    }