problems: add kth smallest element in a sorted matrix

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/kth-smallest-element-in-a-sorted-matrix.cpp

@@ -0,0 +1,52 @@
+#include <algorithm>
+#include <cassert>
+#include <iostream>
+#include <vector>
+
+using namespace std;
+
+class Solution {
+public:
+    int kthSmallest(vector<vector<int>>& matrix, int k)
+    {
+        int low = matrix.front().front();
+        int high = matrix.back().back();
+
+        while (low < high) {
+            int mid = low + (high - low) / 2;
+
+            int rank = 0;
+            for (const auto& row : matrix) {
+                rank += upper_bound(row.begin(), row.end(), mid) - row.begin();
+            }
+
+            if (rank < k) {
+                low = mid + 1;
+            } else {
+                high = mid;
+            }
+        }
+
+        return low;
+    }
+};
+
+int main()
+{
+    Solution s;
+
+    vector<vector<int>> m;
+    int k;
+
+    m = {
+        { 1, 5, 9 },
+        { 10, 11, 13 },
+        { 12, 13, 15 }
+    };
+    k = 8;
+    assert(s.kthSmallest(m, k) == 13);
+
+    m = { { -5 } };
+    k = 1;
+    assert(s.kthSmallest(m, k) == -5);
+}