cherry pick library fix (#2481)

Author: ildar-khisambeev

library/cpp/containers/disjoint_interval_tree/disjoint_interval_tree.h

@@ -132,7 +132,7 @@ class TDisjointIntervalTree {
         }
 
         TIterator completelyRemoveEnd = completelyRemoveBegin != Tree.end() ? Tree.lower_bound(end) : Tree.end();
-        if (completelyRemoveEnd != Tree.end() && completelyRemoveEnd != Tree.begin() && completelyRemoveEnd->first != end) {
+        if (completelyRemoveEnd != Tree.begin() && (completelyRemoveEnd == Tree.end() || completelyRemoveEnd->first != end)) {
             TIterator containingEnd = completelyRemoveEnd;
             --containingEnd;
             if (containingEnd->second > end) {

library/cpp/containers/disjoint_interval_tree/ut/disjoint_interval_tree_ut.cpp

@@ -232,6 +232,18 @@ Y_UNIT_TEST_SUITE(DisjointIntervalTreeTest) {
             UNIT_ASSERT_VALUES_EQUAL(tree.GetNumIntervals(), 2);
             UNIT_ASSERT_VALUES_EQUAL(tree.GetNumElements(), 8);
         }
+
+        // 12. The only one interval
+        {
+            TDisjointIntervalTree<ui64> tree;
+            tree.InsertInterval(1, 10);
+            UNIT_ASSERT_VALUES_EQUAL(tree.GetNumIntervals(), 1);
+            UNIT_ASSERT_VALUES_EQUAL(tree.GetNumElements(), 9);
+            UNIT_ASSERT_VALUES_EQUAL(tree.EraseInterval(0, 6), 5);
+            UNIT_ASSERT_VALUES_EQUAL(tree.GetNumIntervals(), 1);
+            UNIT_ASSERT_VALUES_EQUAL(tree.GetNumElements(), 4);
+            UNIT_ASSERT(tree.Intersects(5, 10));
+        }
     }
 
     Y_UNIT_TEST(IntersectsTest) {