ADD: trapping rain water problem

XDaysChallenge/90DaysChallenge/Day7/trapping_rain_water.cpp

@@ -0,0 +1,31 @@
+// https://leetcode.com/problems/trapping-rain-water/
+
+/**
+ * Keeping leftMax and rightMax as a preprocessing step
+ * Focus on current index, and check if it can store water
+ */
+class Solution {
+public:
+    int trap(vector<int>& height) {
+
+        vector<int> left(height.size(), 0);
+        vector<int> right(height.size(), 0);
+        int leftMax=0, rightMax=0;
+
+        for(int i=0, j=height.size()-1;i<height.size();i++, j--) {
+            left[i] = leftMax;
+            leftMax = max(leftMax, height[i]);
+            right[j] = rightMax;
+            rightMax = max(rightMax, height[j]);
+        }
+
+        int waterStored=0;
+        for(int i=0;i<height.size();i++) {
+            int maxCurrHeight = min(left[i], right[i]);
+            if(maxCurrHeight>height[i]) {
+                waterStored += (maxCurrHeight - height[i]);
+            }
+        }
+        return waterStored;
+    }
+};