feat: leetcode zigzag solution

leetcode/README.md

@@ -10,6 +10,7 @@ LeetCode
 |2|[Add Two Numbers](https://leetcode.com/problems/add-two-numbers/) | [C](./src/2.c)|Medium|
 |3|[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/) | [C](./src/3.c)|Medium|
 |4|[Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays/) | [C](./src/4.c)|Hard|
+|6|[ZigZag conversion](https://leetcode.com/problems/zigzag-conversion/) | [C](./src/4.c)|Medium|
 |7|[Reverse Integer](https://leetcode.com/problems/reverse-integer/) | [C](./src/7.c)|Easy|
 |8|[String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi) | [C](./src/8.c)|Medium|
 |9|[Palindrome Number](https://leetcode.com/problems/palindrome-number/) | [C](./src/9.c)|Easy|

leetcode/src/6.c

@@ -0,0 +1,139 @@
+/**
+ * @file
+ * @brief Implementation of the [ZigZag Conversion](https://leetcode.com/problems/zigzag-conversion/) Leetcode problem
+ * @details
+ * A decent solution to the ZigZag conversion problem.
+ * Take advantage of the fact that the maximum gap between the chars is 2 times
+ * the depth(the number of rows).
+ * The actual gap between the two first chars of a rows depends on the depth of
+ * the row. The gaps between successives chars on the same row is the complement
+ * of the first gap to the maximum gap.
+ * @author [straight_into_the_wall](https://github.com/straight-into-the-wall)
+ */
+
+#include <assert.h>  /// for assert
+#include <stdint.h>  /// for unsigned int with fixed size
+#include <stdio.h>   /// for IO operations
+#include <stdlib.h>  /// for malloc
+#include <string.h>  /// for string tools
+
+/**
+ * @brief Convert a string to the it's zigzag equivalent on a given number of
+ * rows.
+ * @param in the string in input.
+ * @param numRows the desired number of rows.
+ * @returns the converted new (malloced) string.
+ */
+char* convert(char* in, uint16_t numRows)
+{
+    uint16_t len = strlen(in);
+
+    if (len < numRows)
+    {
+        numRows = len;
+    }
+    char* out = calloc(len + 1, sizeof(char));
+
+    if (numRows < 2)
+    {
+        memcpy(out, in, len + 1);
+        return out;
+    }
+
+    uint16_t max = numRows - 1;
+    uint16_t rr = 2 * max;
+    uint16_t i = 0;
+    uint16_t o = 0;
+    uint16_t delta = 0;
+
+    // first row
+    while (i < len)
+    {
+        out[o++] = in[i];
+        i += rr;
+    }
+
+    // middle rows
+    for (uint16_t l = 1; l < max; l++)
+    {
+        i = l;
+        delta = 2 * l;
+        while (i < len)
+        {
+            out[o++] = in[i];
+            delta = rr - delta;
+            i += delta;
+        }
+    }
+
+    // last row
+    i = max;
+    while (i < len)
+    {
+        out[o++] = in[i];
+        i += rr;
+    }
+
+    return out;
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void testZigZag(char* s, int numRows, char* expected)
+{
+    char* ret = convert(s, numRows);
+    int len = strlen(s);
+    int cmp = strncmp(ret, expected, len);
+    assert(!cmp);
+
+    free(ret);
+}
+
+/**
+ * @brief Self-test implementations
+ * @returns void
+ */
+static void test()
+{
+    char* s01 = "PAYPALISHIRING";
+
+    char* r01 = "PINALSIGYAHRPI";
+    testZigZag(s01, 4, r01);
+
+    char* r02 = "PAHNAPLSIIGYIR";
+    testZigZag(s01, 3, r02);
+
+    char* s03 = "A";
+    testZigZag(s03, 1, s03);
+    testZigZag(s03, 3, s03);
+
+    char* s04 =
+        "cbxdwjccgtdoqiscyspqzvuqivzptlpvooynyapgvswoaosaghrffnxnjyeeltzaiznicc"
+        "ozwknwyhzgpqlwfkjqipuu"
+        "jvwtxlbznryjdohbvghmyuiggtyqjtmuqinntqmihntkddnalwnmsxsatqqeldacnnpjfe"
+        "rmrnyuqnwbjjpdjhdeavkn"
+        "ykpoxhxclqqedqavdwzoiorrwwxyrhlsrdgqkduvtmzzczufvtvfioygkvedervvudnegh"
+        "bctcbxdxezrzgbpfhzanff"
+        "eccbgqfmzjqtlrsppxqiywjobspefujlxnmddurddiyobqfspvcoulcvdrzkmkwlyiqdch"
+        "ghrgytzdnobqcvdeqjystm"
+        "epxcaniewqmoxkjwpymqorluxedvywhcoghotpusfgiestckrpaigocfufbubiyrrffmwa"
+        "eeimidfnnzcphkflpbqsvt"
+        "dwludsgaungfzoihbxifoprwcjzsdxngtacw";
+
+    char* r04 =
+        "cbxdwjccgtdoqiscyspqzvuqivzptlpvooynyapgvswoaosaghrffnxnjyeeltzaiznicc"
+        "ozwknwyhzgpqlwfkjqipuu"
+        "jvwtxlbznryjdohbvghmyuiggtyqjtmuqinntqmihntkddnalwnmsxsatqqeldacnnpjfe"
+        "rmrnyuqnwbjjpdjhdeavkn"
+        "ykpoxhxclqqedqavdwzoiorrwwxyrhlsrdgqkduvtmzzczufvtvfioygkvedervvudnegh"
+        "bctcbxdxezrzgbpfhzanff"
+        "eccbgqfmzjqtlrsppxqiywjobspefujlxnmddurddiyobqfspvcoulcvdrzkmkwlyiqdch"
+        "ghrgytzdnobqcvdeqjystm"
+        "epxcaniewqmoxkjwpymqorluxedvywhcoghotpusfgiestckrpaigocfufbubiyrrffmwa"
+        "eeimidfnnzwccpahtkgfnl"
+        "xpdbsqzsjvctwdrwploufdisxgbahuinogzf";
+
+    testZigZag(s04, 472, r04);
+}