diff --git a/CS notes/pages/Leetcode Search-a-2D-Matrix.md b/CS notes/pages/Leetcode Search-a-2D-Matrix.md
index e494bb4..eda580f 100644
--- a/CS notes/pages/Leetcode Search-a-2D-Matrix.md	
+++ b/CS notes/pages/Leetcode Search-a-2D-Matrix.md	
@@ -62,6 +62,7 @@ Binary search algorithm, with simple 2-d to 1-d conversion
 > And use:
 > - l = mid + 1
 > - r = mid - 1
+> Init r as m * n - 1 to avoid mid being OOB.
 ### Solution
 
 ==note how I calculated the mid, i, j, and how I changed r and l==
@@ -72,6 +73,7 @@ public:
     int m = matrix.size();
     int n = matrix[0].size();
     int l = 0;
+    // set r to len - 1 to avoid mid being bigger than len.
     int r = m * n - 1;
     int i; // the key to speed is to precaculate i and j, to save time.
     int j;