力扣1895.最大的幻方
求前缀和暴力枚举幻方边长
- 求行列前缀和
class Solution { public: int largestMagicSquare(vector<vector<int>>& grid) { int n = grid.size() , m = grid[0].size(); vector<vector<int>> rowsum(n,vector<int>(m)); for(int i=0;i<n;i++) { rowsum[i][0] = grid[i][0]; for(int j=1;j<m;j++) rowsum[i][j] = rowsum[i][j-1] + grid[i][j]; } vector<vector<int>> colsum(n,vector<int>(m)); for(int j=0;j<m;j++) { colsum[0][j] = grid[0][j]; for(int i=1;i<n;i++) colsum[i][j] = colsum[i-1][j] + grid[i][j]; } //倒序枚举边长 for(int edge = min(m,n) ; edge>=2;edge--) { for(int i=0;i <= n - edge;i++) { for(int j=0;j<=m - edge;j++) { //求模板 以第一行的和为例 int stdsum = rowsum[i][j + edge - 1] - (j ? rowsum[i][j-1] : 0); bool check = true; for(int ii = i + 1;ii < i + edge; ii ++) { if(rowsum[ii][j + edge - 1] - (j ? rowsum[ii][j - 1] : 0 ) != stdsum) { check = false; break; } if(!check) continue; for(int jj=j;jj<edge + j;jj++) { if(colsum[i + edge - 1][jj] - (i ? colsum[i-1][jj] :0) != stdsum) { check = false; break; } } if(!check) continue; int d1 = 0,d2 = 0; for(int k=0;k<edge;k++) { d1 += grid[i+k][j+k]; d2 += grid[i+k][j+edge-1-k]; } if(d1 == stdsum && d2 == stdsum) return edge; } } } } return 1; } };