修正笔误

notes/Leetcode 题解.md

@@ -127,7 +127,7 @@ public int arrangeCoins(int n) {
     int l = 0, h = n;
     while(l <= h){
         int m = l + (h - l) / 2;
-        long x = m * (m + 1L) / 2;
+        long x = m * (m + 1) / 2;
         if(x == n) return m;
         else if(x < n) l = m + 1;
         else h = m - 1;