added 42 Trapping Rain Water C++

.vscode/settings.json

@@ -0,0 +1,5 @@
+{
+	"files.associations": {
+		"vector": "cpp"
+	}
+}

C++/trapping-rain-water.cpp

@@ -0,0 +1,35 @@
+// https://leetcode.com/problems/trapping-rain-water/
+
+// Idea: 
+// We calculate the prefix and suffix max arrays 
+// The water trapped would be the min of the 2 arrays at a particular index minus the actual height 
+
+class Solution {
+public:
+    int trap(vector<int>& height) {
+        int n = height.size();
+
+        if(n==0) return 0;
+
+        vector<int> prefix(n);
+        vector<int> suffix(n);
+
+        prefix[0] = height[0];
+        for(int i = 1; i<n; i++){
+            prefix[i] = max(prefix[i-1],height[i]);
+        }
+
+        suffix[n-1] = height[n-1];
+        for(int i = n-2; i>=0; i--){
+            suffix[i] = max(suffix[i+1],height[i]);
+        }
+
+        int ans = 0;
+        for(int i = 0; i<n; i++){
+            int x = min(prefix[i],suffix[i])-height[i];
+            if(x>0) ans+=x;         
+        }
+
+        return ans;
+    }
+};

README.md

@@ -387,6 +387,7 @@ Check out ---> [Sample PR](https://github.com/codedecks-in/LeetCode-Solutions/pu
 | 174  | [Dungeon Game](https://leetcode.com/problems/dungeon-game/)                                                         | [C++](./C++/dungeon-game.pp)                              | _O(M\*N)_ | _O(M\*N)_ | Hard       | Dynamic Programming  |      |
 | 070  | [Climbing Stairs](https://leetcode.com/problems/climbing-stairs/)                                                   | [Java](./Java/climbing-stairs.java)                       | _O(N)_    | _O(1)_    | Easy       | DP                   |      |
 | 730  | [Count Different Palindromic Subsequences](https://leetcode.com/problems/count-different-palindromic-subsequences/) | [C++](./C++/Count-Different-Palindromic-Subsequences.cpp) | _O(N\*N)_ | _O(N\*N)_ | Hard       | DP                   |      |
+| 42  | [Trapping Rain Water](https://leetcode.com/problems/trapping-rain-water/) | [C++](./C++/trapping-rain-water.cpp) | _O(N)_ | _O(N)_ | Hard       | DP                   |      |
 
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