chen-w
7/26/2017 - 10:29 PM

BFS template

BFS template

public class Solution {
  public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
    ArrayList result = ArrayList(); // result's element is level
    
    if (root == null) {
      return result;
    }
    
    Queue<TreeNode> queue = new LinkedList<TreeNode>(); 
    // queue is an interface so need to use LinkedList or ArrayDeque... to implement it
    queue.offer(root);
    
    while (!queue.isEmpty()) {
      ArrayList<Integer> level = new ArrayList<Integer>();
      int size = queue.size();
      for (int i = 0; i < size; i++) {
        TreeNode head = queue.poll();
        level.add(head.val);
        if (head.left != null) {
          queue.offer(head.left);
        }
        if (head.right != null) {
          queue.offer(head.right);
        }
      }
      result.add(level);
    }
    
    return result;
  }  
}