add README

Author: ductnn

leetcode/topic/B-Tree/515/README.md

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+## [515. Find Largest Value in Each Tree Row]((https://leetcode.com/problems/find-largest-value-in-each-tree-row/description/))
+
+Given the root of a binary tree, return an array of the largest value in each
+row of the tree (0-indexed).
+
+Ex1:
+![alt text](image.png)
+Input: root = [1,3,2,5,3,null,9]
+Output: [1,3,9]
+
+Ex2:
+Input: root = [1,2,3]
+Output: [1,3]
+
+### Approach
+
+**Breadth-First Search (BFS)**
+- We can use BFS to traverse the tree level by level. We will use a queue to store
+the nodes at each level.
+- For each level, we will find the maximum value and add it to the result array.
+- We will repeat this process until we have traversed all levels of the tree.