Leetcode Algorithm

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]
]

Solution:

// create a result array, a queue array and a row array.
// queue.push() and queue.shift()
var levelOrder = function(root) {
  let result = [];
  if (root === null) return result; // return an empty array
  let queue = [];
  queue.push(root);
  while (queue.length > 0){
    let row = [];
    let rowSize = queue.length;
    while (rowSize > 0){
      let currentNode = queue.shift();
      if (currentNode.left !== null){
          queue.push(currentNode.left);
      }
      if (currentNode.right !== null){
          queue.push(currentNode.right);
      }
      // put the nodes in the queue to the row
      row.push(currentNode.val);
      rowSize --;
    }
    result.push(row);
  }
  return result;
};

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