Added rmq query

src/algorithms/rmq/rmq_segment_tree.h

@@ -0,0 +1,66 @@
+#ifndef RMQ_SEGMENT_TREE_H
+#define RMQ_SEGMENT_TREE_H
+
+#ifdef __cplusplus
+#include <iostream>
+#include <vector>
+#include <cassert>
+#endif
+
+
+/**
+* @brief credits to @neal_wu for his RMQ query
+* RMQ struct for range query minimum
+*/
+template<typename T, bool maximum_mode = false>
+struct RMQ {
+    static int highest_bit(unsigned x) {
+        return x == 0 ? -1 : 31 - __builtin_clz(x);
+    }
+
+    int n = 0;
+    std::vector<T> values;
+    std::vector<std::vector<int>> range_low;
+
+    RMQ(const std::vector<T> &_values = {}) {
+        if (!_values.empty())
+            build(_values);
+    }
+
+    // Note: when `values[a] == values[b]`, returns b.
+    // Need to change this if you want to return a instead of b
+    int better_index(int a, int b) const {
+        return (maximum_mode ? values[b] < values[a] : values[a] < values[b]) ? a : b;
+    }
+
+    void build(const std::vector<T> &_values) {
+        values = _values;
+        n = int(values.size());
+        int levels = highest_bit(n) + 1;
+        range_low.resize(levels);
+
+        for (int k = 0; k < levels; k++)
+            range_low[k].resize(n - (1 << k) + 1);
+
+        for (int i = 0; i < n; i++)
+            range_low[0][i] = i;
+
+        for (int k = 1; k < levels; k++)
+            for (int i = 0; i <= n - (1 << k); i++)
+                range_low[k][i] = better_index(range_low[k - 1][i], range_low[k - 1][i + (1 << (k - 1))]);
+    }
+
+    // Note: breaks ties by choosing the largest index.
+    int query_index(int a, int b) const {
+        assert(0 <= a && a < b && b <= n);
+        int level = highest_bit(b - a);
+        return better_index(range_low[level][a], range_low[level][b - (1 << level)]);
+    }
+
+    T query_value(int a, int b) const {
+        return values[query_index(a, b)];
+    }
+};
+
+
+#endif

tests/algorithms/rmq/rmq_segment_tree.cc

@@ -0,0 +1,20 @@
+#include "../../../src/algorithms/rmq/rmq_segment_tree.h"
+#include "../../../third_party/catch.hpp"
+
+TEST_CASE("Testing rmq 1") {
+    std::vector<int> v {1, 5, 4, 2, 3, 7};
+    RMQ<int> rr(v);
+
+    REQUIRE(rr.query_value(0, 4) == 1);
+    REQUIRE(rr.query_value(0, 3) == 1);
+    REQUIRE(rr.query_value(0, 2) == 1);
+}
+
+TEST_CASE("Testing rmq 2") {
+    std::vector<int> v {-1, -2, -3, -4, -5, -6};
+
+    RMQ<int> rr(v);
+    REQUIRE(rr.query_value(0, 1) == -1);
+    REQUIRE(rr.query_value(0, 2) == -2);
+    REQUIRE(rr.query_value(0, 3) == -3);
+}