Add subarray sum equals K flashcards (LC 560, 974, 325)

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@@ -81,3 +81,19 @@ public:
     }
 };
 #+end_src
+
+
+* TODO study: why min doesn't work with Fenwick tree (no inverse, update breaks on increase)
+
+* Binary search
+#+begin_src python
+def bs(a, x, l, r):
+    if l >= r:
+        return l
+
+    m = l + (r-l) // 2
+    if (a[m] < x):
+        return bs(a, x, m+1, r);
+    else:
+        return bs(a, x, l, m)
+#+end_src