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]
]

二叉树的层级遍历 写法主要分两种 1个queue和2个queue

2个queue

public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
if(root == null) return res;

Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
while (!queue.isEmpty()) {
Queue<TreeNode> queue2 = new LinkedList<TreeNode>();
List<Integer> list = new ArrayList<Integer>();
while (!queue.isEmpty()) {
TreeNode node = queue.poll();
list.add(node.val);
if (node.left != null) queue2.offer(node.left);
if (node.right != null) queue2.offer(node.right);
}
res.add(list);
queue = queue2;
}
return res;
}

1个queue 各层之间用null分隔

public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new ArrayList<List<Integer>>();
if(root == null) return res;

Queue<TreeNode> queue = new LinkedList<TreeNode>();
queue.offer(root);
queue.offer(null);
List<Integer> list = new ArrayList<Integer>();
while (!queue.isEmpty()) {
TreeNode node = queue.poll();
if (node == null) {
res.add(list);
list = new ArrayList<Integer>();
if (!queue.isEmpty()) queue.offer(null);
} else {
list.add(node.val);
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
}
}
return res;
}

下面这个思路也很厉害 1个queue 但是没有添加null

public List<List<Integer>> levelOrder(TreeNode root) {
Queue<TreeNode> queue = new LinkedList<TreeNode>();
List<List<Integer>> wrapList = new LinkedList<List<Integer>>();

if(root == null) return wrapList;

queue.offer(root);
while(!queue.isEmpty()){
int levelNum = queue.size();
List<Integer> subList = new LinkedList<Integer>();
for(int i=0; i<levelNum; i++) {
if(queue.peek().left != null) queue.offer(queue.peek().left);
if(queue.peek().right != null) queue.offer(queue.peek().right);
subList.add(queue.poll().val);
}
wrapList.add(subList);
}
return wrapList;
}