Add "Top 10 Algorithms" examples

example/CMakeLists.txt

@@ -42,3 +42,15 @@ add_example(example-docs-set-union docs/set_union.cpp)
 add_example(example-docs-scan-first docs/scan_first.cpp)
 add_example(example-docs-starts-with docs/starts_with.cpp)
 add_example(example-docs-unfold docs/unfold.cpp)
+
+add_example(example-top10-rain-water top10/01_trapping_rain_water.cpp)
+add_example(example-top10-mco top10/02_max_consecutive_ones.cpp)
+add_example(example-top10-lcis top10/03_longest_continuous_increasing_subsequence.cpp)
+add_example(example-top10-kadanes top10/04_maximum_subarray_sum.cpp)
+add_example(example-top10-sushi top10/05_sushi_for_two.cpp)
+add_example(example-top10-max-gap top10/06_max_gap.cpp)
+add_example(example-top10-max-gap-count top10/07_max_gap_count.cpp)
+add_example(example-top10-tco top10/08_three_consecutive_odds.cpp)
+add_example(example-top10-skyline top10/09_skyline.cpp)
+add_example(example-top10-ocean-view top10/10_ocean_view.cpp)
+

example/top10/01_trapping_rain_water.cpp

@@ -0,0 +1,43 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * Given n non-negative integers representing an elevation map where the width
+ * of each bar is 1, compute how much water it can trap after raining.
+ *
+ * https://leetcode.com/problems/trapping-rain-water/
+ *
+ */
+
+#include <flux.hpp>
+
+#include <algorithm>
+
+auto const rain_water = [](std::initializer_list<int> heights)
+{
+    // Find the index of the maximum height
+    flux::cursor auto max_idx = flux::find_max(heights);
+
+    // Split the input sequence into two at the max position
+    auto left = flux::slice(heights, flux::first(heights), max_idx);
+    auto right = flux::slice(heights, max_idx, flux::last(heights));
+
+    // To calculate the trapped rain water for each half, we sum up the
+    // difference between each element and the maximum seen up to that point
+    auto trapped = [](flux::sequence auto seq) {
+        return flux::zip(flux::scan(seq, flux::cmp::max), seq)
+                .map(flux::unpack(std::minus{}))
+                .sum();
+    };
+
+    // The final answer is the total of the trapped rain water reading
+    // left-to-right for the left half, and right-to-left for the right half
+    return trapped(left) + trapped(flux::reverse(right));
+};
+
+static_assert(rain_water({0,1,0,2,1,0,1,3,2,1,2,1}) == 6);
+static_assert(rain_water({4,2,0,3,2,5}) == 9);
+
+int main() {}

example/top10/02_max_consecutive_ones.cpp

@@ -0,0 +1,45 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * Given a binary array nums, return the maximum number of consecutive 1's in the array.
+ *
+ * https://leetcode.com/problems/max-consecutive-ones/
+ */
+
+#include <flux.hpp>
+
+namespace version1 {
+
+auto const mco = [](std::initializer_list<int> nums)
+{
+    return flux::ref(nums)
+            .chunk_by(std::equal_to{})
+            .map(flux::sum)
+            .max()
+            .value_or(0);
+};
+
+static_assert(mco({1,1,0,1,1,1}) == 3);
+static_assert(mco({1,0,1,1,0,1}) == 2);
+
+}
+
+namespace version2 {
+
+auto const mco = [](std::initializer_list<int> nums)
+{
+    return flux::ref(nums)
+            .scan([](int count, int i) { return i * (count + i); })
+            .max()
+            .value_or(0);
+};
+
+static_assert(mco({1,1,0,1,1,1}) == 3);
+static_assert(mco({1,0,1,1,0,1}) == 2);
+
+}
+
+int main() {}

example/top10/03_longest_continuous_increasing_subsequence.cpp

@@ -0,0 +1,29 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * Given an unsorted array of integers nums, return the length of the longest
+ * continuous increasing subsequence (i.e. subarray). The subsequence must be
+ * strictly increasing.
+ *
+ * https://leetcode.com/problems/longest-continuous-increasing-subsequence/
+ */
+
+#include <flux.hpp>
+
+auto const lcis = [](std::initializer_list<int> nums)
+{
+    return flux::ref(nums)
+            .chunk_by(std::less{})
+            .map(flux::count)
+            .max()
+            .value_or(0);
+};
+
+static_assert(lcis({}) == 0);
+static_assert(lcis({1, 3, 5, 4, 7}) == 3); // [1, 3, 5]
+static_assert(lcis({2, 2, 2, 2, 2}) == 1); // [2] (must be strictly increasing)
+
+int main() {}

example/top10/04_maximum_subarray_sum.cpp

@@ -0,0 +1,28 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * Given an integer array nums, find the subarray with the largest sum, and
+ * return its sum.
+ *
+ * https://leetcode.com/problems/maximum-subarray/
+ *
+ */
+
+#include <flux.hpp>
+
+auto const kadanes = [](std::initializer_list<int> nums)
+{
+    return flux::ref(nums)
+            .scan([](int sum, int i) { return std::max(i, sum + i); })
+            .max()
+            .value_or(0);
+};
+
+static_assert(kadanes({-2, 1, -3, 4, -1, 2, 1, -5, 4}) == 6);
+static_assert(kadanes({1}) == 1);
+static_assert(kadanes({5, 4, -1, 7, 8}) == 23);
+
+int main() {}

example/top10/05_sushi_for_two.cpp

@@ -0,0 +1,27 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * https://codeforces.com/contest/1138/problem/A
+ */
+
+#include <flux.hpp>
+
+auto const sushi_for_two = [](std::initializer_list<int> sushi)
+{
+    return 2 * flux::ref(sushi)
+                .chunk_by(std::equal_to{})
+                .map(flux::count)
+                .pairwise_map(flux::cmp::min)
+                .max()
+                .value_or(0);
+};
+
+static_assert(sushi_for_two({}) == 0);
+static_assert(sushi_for_two({2, 2, 2, 1, 1, 2, 2}) == 4);
+static_assert(sushi_for_two({1, 2, 1, 2, 1, 2}) == 2);
+static_assert(sushi_for_two({2, 2, 1, 1, 1, 2, 2, 2, 2}) == 6);
+
+int main() {}

example/top10/06_max_gap.cpp

@@ -0,0 +1,39 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * https://leetcode.com/problems/maximum-gap/
+ *
+ * Note that the original problem specifies that the solution should have O(N)
+ * runtime and use O(N) extra space. We instead use an O(NlogN) solution as
+ * discussed on ADSP episode 115 (https://adspthepodcast.com/2023/02/03/Episode-115.html)
+ * The NlogN version still passes the time limit constraints on leetcode, however.
+ */
+
+#include <flux.hpp>
+
+#include <cassert>
+
+namespace {
+
+// std::abs is not constexpr in C++20
+auto const abs = [](std::signed_integral auto i) { return i < 0 ? -i : i; };
+
+auto const max_gap = [](std::vector<int> nums)
+{
+    flux::sort(nums);
+    return flux::ref(nums)
+            .pairwise_map([](int a, int b) { return abs(a - b); })
+            .max()
+            .value_or(0);
+};
+
+}
+
+int main()
+{
+    assert(max_gap({3, 6, 9, 1}) == 3);
+    assert(max_gap({10}) == 0);
+}

example/top10/07_max_gap_count.cpp

@@ -0,0 +1,91 @@
+
+// Copyright (c) 2023 Tristan Brindle (tcbrindle at gmail dot com)
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+/*
+ * https://theweeklychallenge.org/blog/perl-weekly-challenge-198/
+ *
+ * Also discussed on ADSP episode 116 (https://adspthepodcast.com/2023/02/03/Episode-116.html)
+ */
+
+#include <flux.hpp>
+
+#include <cassert>
+
+// GCC 11 doesn't support constexpr std::vector
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 12
+#define COMPILER_IS_GCC11
+#endif
+
+// std::abs is not constexpr in C++20
+auto const c_abs = [](std::signed_integral auto i) { return i < 0 ? -i : i; };
+
+namespace version1 {
+
+// Sort + two passes
+auto const max_gap_count = [](std::vector<int> nums)
+{
+    flux::sort(nums);
+
+    auto diffs = flux::ref(nums).pairwise_map([](int a, int b) { return c_abs(a - b); });
+
+    return diffs.count_eq(diffs.max().value_or(0));
+};
+
+#ifndef COMPILER_IS_GCC11
+static_assert(max_gap_count({2, 5, 8, 1}) == 2);
+static_assert(max_gap_count({3, 6, 9, 1}) == 2);
+static_assert(max_gap_count({10}) == 0);
+#endif
+
+}
+
+namespace version2 {
+
+// Sort + one pass
+auto const max_gap_count = [](std::vector<int> nums)
+{
+    struct max_count {
+        int value;
+        int count;
+    };
+
+    flux::sort(nums);
+    return flux::ref(nums)
+            .pairwise_map([](int a, int b) { return c_abs(a - b); })
+            .fold([](max_count max, int i) {
+                if (i > max.value) {
+                    max = max_count{i, 1};
+                } else if (i == max.value ){
+                    ++max.count;
+                }
+                return max;
+            }, max_count{})
+            .count;
+};
+
+#ifndef COMPILER_IS_GCC11
+static_assert(max_gap_count({2, 5, 8, 1}) == 2);
+static_assert(max_gap_count({3, 6, 9, 1}) == 2);
+static_assert(max_gap_count({10}) == 0);
+#endif
+
+}
+
+int main()
+{
+    {
+        using namespace version1;
+        assert(max_gap_count({2, 5, 8, 1}) == 2);
+        assert(max_gap_count({3, 6, 9, 1}) == 2);
+        assert(max_gap_count({10}) == 0);
+    }
+
+    {
+        using namespace version2;
+        assert(max_gap_count({2, 5, 8, 1}) == 2);
+        assert(max_gap_count({3, 6, 9, 1}) == 2);
+        assert(max_gap_count({10}) == 0);
+    }
+}