feat: add trapping rain water (#1132)

* add trapping rain water.

* add short description of algorithm

* substitute min/max with define

* fix directory DIRECTORY.md

* chore: apply suggestions from code review

Co-authored-by: David Leal <halfpacho@gmail.com>

leetcode/DIRECTORY.md

@@ -26,6 +26,7 @@
 | 29   | [Divide Two Integers](https://leetcode.com/problems/divide-two-integers/)                                                       | [C](./src/29.c)   | Medium     |
 | 35   | [Search Insert Position](https://leetcode.com/problems/search-insert-position/)                                                 | [C](./src/35.c)   | Easy       |
 | 38   | [Count and Say](https://leetcode.com/problems/count-and-say/)                                                                   | [C](./src/38.c)   | Easy       |
+| 42   | [Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/)                                                       | [C](./src/42.c)   | Hard       |
 | 53   | [Maximum Subarray](https://leetcode.com/problems/maximum-subarray/)                                                             | [C](./src/53.c)   | Easy       |
 | 62   | [Unique Paths](https://leetcode.com/problems/unique-paths/description/)                                                         | [C](./src/62.c)   | Medium     |
 | 66   | [Plus One](https://leetcode.com/problems/plus-one/)                                                                             | [C](./src/66.c)   | Easy       |

leetcode/src/42.c

@@ -0,0 +1,27 @@
+#define max(x,y)(((x)>(y))?(x):(y))
+#define min(x,y)(((x)<(y))?(x):(y))
+
+// Max stack. Runtime: O(n), Space: O(n)
+// Algorithm description: 
+// - Calculate the stack of maximums from right board.
+// - For each index find left maximum and right maximum of height
+// - The each index if heights could place nor greater than minimum of left and right max minus curr height
+// - Sum all index in result
+int trap(int* height, int heightSize){
+    int* rightMaxStack = malloc(heightSize * sizeof(int));
+    rightMaxStack[heightSize - 1] = height[heightSize - 1];
+
+    for (int i = heightSize - 2; i >= 0; i--){
+        rightMaxStack[i] = max(rightMaxStack[i + 1], height[i]);
+    }
+
+    int leftMax = 0;
+    int result = 0;
+    for (int i = 0; i < heightSize; i++){
+        leftMax = max(leftMax, height[i]);
+        result += max(0, min(leftMax, rightMaxStack[i]) - height[i]);
+    }
+    
+    free(rightMaxStack);
+    return result;
+}