3. Find Level Order Traversal of a Binary Tree.

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]
]

Solution

class Solution {
 public List<List<Integer>> levelOrder(TreeNode root) {
 
 List<List<Integer>> sol = new ArrayList<>();
 
 if (root == null) {
 return sol;
 } 
 
 Queue<TreeNode> q = new LinkedList<>();
 q.offer(root);
 
 while(!q.isEmpty()){
 
 int levelSize = q.size();
 List<Integer> level = new ArrayList<>();
 
 while(levelSize → 0){
 TreeNode t = q.poll();
 level.add(t.val);
 
 if(t.left != null){
 q.offer(t.left);
 }
 
 if(t.right != null) {
 q.offer(t.right);
 }
 
 }
 
 sol.add(level);
 
 }
 
 return sol;
 }
}