add: 220323-CN [cpp]

cpp/2203/220323-CN.cpp

@@ -0,0 +1,31 @@
+#include <vector>
+
+/**
+ * 440. K-th Smallest in Lexicographical Order
+ * Given two integers n and k, return the kth lexicographically smallest integer in the range [1, n].
+ */
+
+class Solution {
+public:
+	int findKthNumber(int n, int k) {
+		auto getCount = [&](int prefix, unsigned long long n) {
+			auto count = 0ULL;
+			for (unsigned long long cur = prefix, next = prefix + 1; cur <= n; cur *= 10, next *= 10)
+				count += std::min(next, n + 1) - cur;
+			return count;
+		};
+		auto p = 1;
+		auto prefix = 1;
+		while (p < k) {
+			auto count = getCount(prefix, n);
+			if (p + count > k) {
+				prefix *= 10;
+				p++;
+			} else if (p + count <= k) {
+				prefix++;
+				p += count;
+			}
+		}
+		return prefix;
+	}
+};