HARD

Author: SjxSubham

407. Trapping Rain Water II

@@ -0,0 +1,55 @@
+class Solution {
+public:
+       typedef pair<int, pair<int, int>> PP;
+    vector<vector<int>> directions = {{0, 1}, {1, 0}, {0, -1}, {-1, 0}};
+
+    int trapRainWater(vector<vector<int>>& heightMap) {
+
+        int m = heightMap.size();
+        int n = heightMap[0].size();
+
+        priority_queue<PP, vector<PP>, greater<>> boundaryCells;
+
+        vector<vector<bool>> visited(m, vector<bool>(n, false));
+
+        for (int r = 0; r < m; ++r) {
+            for (int c : {0, n - 1}) { //0 : left most boundary, cols-1 right most boundary
+                boundaryCells.push({heightMap[r][c], {r, c}});
+                visited[r][c] = true;
+            }
+        }
+
+        for (int c = 0; c < n; ++c) {
+            for (int r : {0, m - 1}) { //0 : top most boundary, rows-1 bottom most boundary
+                boundaryCells.push({heightMap[r][c], {r, c}});
+                visited[r][c] = true;
+            }
+        }
+
+        int water = 0;
+
+        while(!boundaryCells.empty()){
+            PP p = boundaryCells.top();
+            boundaryCells.pop();
+
+            int heigth = p.first;
+            int i = p.second.first;
+            int j = p.second.second;
+
+            //find neighbourss
+            for(vector<int>& dir: directions){
+                int i_ = i + dir[0];
+                int j_ = j + dir[1];
+
+                if(i_ >= 0 && i_ <m && j_ >= 0 && j_ <n && !visited[i_][j_]){
+                    water += max(heigth -  heightMap[i_][j_], 0);
+                    boundaryCells.push({max(heigth, heightMap[i_][j_]), {i_, j_}});
+
+                    visited[i_][j_] = true;
+
+                }
+            }
+        }
+        return water;
+    }
+};