Update 850.Rectangle-Area-II_v2.cpp

Others/850.Rectangle-Area-II/850.Rectangle-Area-II_v2.cpp

@@ -1,5 +1,44 @@
-typedef long long ll;
+using ll = long long;
+class Diff2d {    
+public:
+    vector<vector<int>>f;
+    vector<vector<int>>diff;    
+    int m,n;
+    Diff2d(int m, int n)
+    {
+        this->m = m;
+        this->n = n;
+        diff.resize(m+1);
+        f.resize(m+1);        
+        for (int i=0; i<m+1; i++)
+        {
+            diff[i].resize(n+1);
+            f[i].resize(n+1);
+        }            
+    }
+    void set(int x0, int y0, int x1, int y1, int val)
+    {
+        diff[x0][y0]+=val;
+        diff[x0][y1+1]-=val;
+        diff[x1+1][y0]-=val;
+        diff[x1+1][y1+1]+=val;
+    }
+    void compute()
+    {
+        f[0][0] = diff[0][0];
+        for (int i=0; i<m; i++)
+            for (int j=0; j<n; j++)
+            {
+                int a = i==0?0:f[i-1][j];
+                int b = j==0?0:f[i][j-1];
+                int c = (i==0||j==0)?0:f[i-1][j-1];                
+                f[i][j] = a + b - c + diff[i][j];
+            }
+    }    
+};
+
 class Solution {
+    ll M = 1e9+7;
 public:
     int rectangleArea(vector<vector<int>>& rectangles) 
     {
@@ -24,37 +63,30 @@ class Solution {
             Y2idx[col[i]] = i;
 
         int m = row.size(), n = col.size();
-        vector<vector<ll>>sum(m, vector<ll>(n));
-        vector<vector<ll>>diff(m, vector<ll>(n));
-
+        Diff2d grid(m,n);
+
         for (auto rect: rectangles)
         {
-            ll x0 = rect[0];
-            ll y0 = rect[1];
-            ll x1 = rect[2];
-            ll y1 = rect[3];            
-            diff[X2idx[x0]][Y2idx[y0]]+=1;
-            diff[X2idx[x0]][Y2idx[y1]]-=1;
-            diff[X2idx[x1]][Y2idx[y0]]-=1;
-            diff[X2idx[x1]][Y2idx[y1]]+=1;            
+            int i = X2idx[rect[0]];
+            int j = Y2idx[rect[1]];
+            int x = X2idx[rect[2]]-1;
+            int y = Y2idx[rect[3]]-1;
+            grid.set(i,j,x,y,1);
         }
-
-        
+        grid.compute();
+
         ll ret = 0;
-        ll M = 1e9+7;
-        sum[0][0] = diff[0][0];
-        if (diff[0][0]>0)
-            ret += (row[1]-row[0])*(col[1]-col[0])%M;
-
         for (int i=0; i<m-1; i++)
             for (int j=0; j<n-1; j++)
             {
-                if (i==0 && j==0) continue;                                 
-                sum[i][j] = (i>=1?sum[i-1][j]:0) + (j>=1?sum[i][j-1]:0) - ((i>=1&&j>=1)?sum[i-1][j-1]:0) + diff[i][j];                                
-                if (sum[i][j] > 0)
-                    ret = (ret + (row[i+1]-row[i])*(col[j+1]-col[j])) % M;                                
+                if (grid.f[i][j]>0)
+                {
+                    ll dx = row[i+1]-row[i];
+                    ll dy = col[j+1]-col[j];
+                    ret += dx*dy%M;
+                    ret %= M;
+                }
             }
-
         return ret;
     }
 };