力扣1878.矩阵中最大的三个菱形和

• 斜前缀和

  • 遍历矩阵元素，同时求当前点左下右下两位置的前缀和
  • 枚举每个菱形中心，遍历边长

•   int sum1[101][101];int sum2[101][101];class Solution {public:vector<int> getBiggestThree(vector<vector<int>>& grid) {int n = grid.size() , m = grid[0].size();for(int i=1;i<=n;i++)for(int j=1;j<=m;j++){sum1[i][j] = sum1[i-1][j-1] + grid[i-1][j-1];sum2[i][j] = sum2[i-1][j+1] + grid[i-1][j-1];}set<int> S;for(int i=1;i<=n;i++)for(int j=1;j<=m;j++){S.insert(grid[i-1][j-1]);for(int k=1;i + k <= n && i - k >= 1 && j + k <= m && j - k >= 1;k++){int a = sum1[i+k][j] - sum1[i][j-k];int b = sum1[i][j+k] - sum1[i-k][j];int c = sum2[i + k][j] - sum2[i][j + k];int d = sum2[i][j - k] - sum2[i - k][j];S.insert(a+b+c+d - grid[i+k-1][j-1] + grid[i-k-1][j-1]);}while(S.size() > 3) S.erase(S.begin());}return vector<int> (S.rbegin(),S.rend());}};