Merge pull request codewithdev#42 from alpha-neutr0n/master

Create trap_rain_water.cpp

Author: codewithdev

10- Recursion & Backtracking/trap_rain_water.cpp

@@ -0,0 +1,33 @@
+//Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
+
+class Solution
+{
+public:
+    int trap(vector<int>& height)
+    {
+        if (height.size() < 3) { return 0; }
+        
+        // Get the indices of two highest bars, sort them and put into top2bars
+        array<vector<int>::iterator, 2> top2bars;
+        top2bars[0] = max_element(height.begin(), height.end());
+        int tmp = *(top2bars[0]);
+        *(top2bars[0]) = INT_MIN;
+        top2bars[1] = max_element(height.begin(), height.end());
+		int top2bars_min = *(top2bars[1]); 
+        *(top2bars[0]) = tmp;
+        sort(top2bars.begin(), top2bars.end());
+        
+		// Calculate the amount of water between the two highest bars
+        int output = 0;  // amount of trapped water
+        for (auto it = top2bars[0] + 1; it < top2bars[1]; ++it)
+        {
+            output += top2bars_min - *it;
+        }
+        
+		// Call recursively trap() on bars left and right to the top2bars and add to the output
+        vector<int>&& left = {height.begin(), top2bars[0] + 1};
+        vector<int>&& right = {top2bars[1], height.end()};
+            
+        return output + trap(left) + trap(right);
+    }
+};