Problem 102 (#26)

* add solution and resources

* update readme

Author: curtisbarnard

README.md

@@ -39,7 +39,7 @@ As I work through this list I figure I would make a GitHub repo with my solution
 28. [K Closest Points to Origin #973](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/k-closest-origin-973.md)
 29. [Longest Substring Without Repeating Characters #3](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/longest-substring-3.md)
 30. [3Sum #15](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/3Sum-15.md)
-31. Binary Tree Level Order Traversal #102
+31. [Binary Tree Level Order Traversal #102](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/binary-tree-level-102.md)
 32. Clone Graph #133
 33. Evaluate Reverse Polish Notation #150
 34. Course Schedule #207
@@ -112,6 +112,7 @@ In order to practice with similar data structures I'll be placing each problem i
 - [Flood Fill #733](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/flood-fill-733.md)
 - [Implement Queue using Stacks #232](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/implement-queue-stacks-232.md)
 - [Maximum Depth of Binary Tree #104](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/depth-binary-tree-104.md)
+- [Binary Tree Level Order Traversal #102](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/binary-tree-level-102.md)
 
 ### Stack
 
@@ -150,6 +151,7 @@ In order to practice with similar data structures I'll be placing each problem i
 - [Balanced Binary Tree #110](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/balanced-binary-tree-110.md)
 - [Diameter of Binary Tree #543](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/diameter-binary-tree-543.md)
 - [Maximum Depth of Binary Tree #104](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/depth-binary-tree-104.md)
+- [Binary Tree Level Order Traversal #102](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/binary-tree-level-102.md)
 
 ### Heap
 
@@ -183,6 +185,7 @@ Within the problems above there are several patterns that often occur. I plan to
 - [Flood Fill #733](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/flood-fill-733.md)
 - [Maximum Depth of Binary Tree #104](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/easy/depth-binary-tree-104.md)
 - [01 Matrix #542](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/01-matrix-542.md)
+- [Binary Tree Level Order Traversal #102](https://github.com/curtisbarnard/leetcode-grind75-javascript/blob/main/medium/binary-tree-level-102.md)
 
 ### Depth First Search
 

medium/binary-tree-level-102.md

@@ -0,0 +1,105 @@
+# Binary Tree Level Order Traversal
+
+Page on leetcode: https://leetcode.com/problems/binary-tree-level-order-traversal/
+
+## Problem Statement
+
+Given the root of a binary tree, return the level order traversal of its nodes' values. (i.e., from left to right, level by level).
+
+### Constraints
+
+- The number of nodes in the tree is in the range [0, 2000].
+- -1000 <= Node.val <= 1000
+
+### Example
+
+```
+Input: root = [3,9,20,null,null,15,7]
+Output: [[3],[9,20],[15,7]]
+```
+
+## Solution
+
+- BFS with a queue
+- Only need to return the values
+- Need to track the level so that once a level is done a new array is created
+- Create a result array
+
+### Pseudocode
+
+1. Create a result array
+2. Create a queue
+3. Add root to queue with level 1
+4. Loop while queue is not empty
+5. Loop on level
+6. Dequeue and push value to level array
+7. If node.left is valid push to queue, same with right with level as curlevel + 1
+8. Once level is done push level array to result array
+9. Increment level
+10. Return result
+
+### Initial Solution
+
+This solution has a time and space complexity O(n).
+
+```javascript
+const levelOrder = function (root) {
+  const result = [];
+  // Handle if root is null
+  if (root) {
+    const queue = [[root, 1]];
+    let level = 1;
+    while (queue.length > 0) {
+      const levelArr = [];
+      // Looping on current level
+      while (queue.length > 0 && level === queue[0][1]) {
+        const node = queue.shift();
+        levelArr.push(node[0].val);
+        if (node[0].left) {
+          queue.push([node[0].left, node[1] + 1]);
+        }
+        if (node[0].right) {
+          queue.push([node[0].right, node[1] + 1]);
+        }
+      }
+      // Add everything from current level to result
+      result.push(levelArr);
+      level++;
+    }
+  }
+  return result;
+};
+```
+
+### Optimized Solution
+
+This solution has a time and space complexity O(n). It's pretty much the same as above just using a cleaner method to track each level. You can see an explanation of the approach here: https://www.youtube.com/watch?v=6ZnyEApgFYg
+
+```javascript
+const levelOrder = function (root) {
+  const result = [];
+  // Handle if root is null
+  if (root) {
+    const queue = [root];
+    while (queue.length > 0) {
+      const qLen = queue.length;
+      const levelArr = [];
+      // Looping on current level
+      for (let i = 0; i < qLen; i++) {
+        const node = queue.shift();
+        // queue will have null nodes. If nodes are null we just skip them.
+        if (node) {
+          levelArr.push(node.val);
+          queue.push(node.left);
+          queue.push(node.right);
+        }
+      }
+      // Add everything from current level to result is it is non empty
+      if (levelArr.length > 0) {
+        result.push(levelArr);
+      }
+    }
+  }
+  return result;
+};
+```