【Leetcode】Binary Tree Level Order Traversal (Tree 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,#,#,15,7},

    3   / \  9  20    /  \   15   7

return its level order traversal as:

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

这道题让层次遍历，所以最先想到的一定是广度搜索，我们用一个queue来放节点，一个value来放值，每一次一个queue的根节点可以滋生出2个子节点，这样层层推进，这道题最后就能得到答案

public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();if (root == null)return result;LinkedList<TreeNode> queue = new LinkedList<TreeNode>();ArrayList<Integer> value = new ArrayList<Integer>();queue.add(root);int remainNum = 1;int curNum = 0;while (!queue.isEmpty()) {TreeNode cur = queue.remove();value.add(cur.val);remainNum--;if (cur.left != null) {queue.add(cur.left);curNum++;}if (cur.right != null) {queue.add(cur.right);curNum++;}if (remainNum == 0) {remainNum = curNum;curNum = 0;result.add(value);value = new ArrayList<Integer>();}}return result;}