dp

分别计算从左到右、从右到左、从上到下、从下到上4个方向可能的值，然后计算所有为‘0’的地方的4个方向的值的最大值

 https://www.cnblogs.com/grandyang/p/5599289.html

class Solution {
public:
    /**
     * @param grid: Given a 2D grid, each cell is either 'W', 'E' or '0'
     * @return: an integer, the maximum enemies you can kill using one bomb
     */
    int maxKilledEnemies(vector<vector<char> > &grid) {
        // write your code here
        int m = grid.size();
        if(m <= 0)
            return 0;
        int n = grid[0].size();
        if(n <= 0)
            return 0;
        vector<vector<int> > left(m,vector<int>(n,0)),right(m,vector<int>(n,0)),top(m,vector<int>(n,0)),down(m,vector<int>(n,0));
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                int tmp;
                if(j == 0 || grid[i][j] == 'W')
                    tmp = 0;
                else
                    tmp = left[i][j-1];
                if(grid[i][j] == 'E')
                    left[i][j] = tmp + 1;
                else
                    left[i][j] = tmp;
            }
        }
        for(int i = 0;i < m;i++){
            for(int j = n - 1;j >= 0;j--){
                int tmp;
                if(j == n - 1 || grid[i][j] == 'W')
                    tmp = 0;
                else
                    tmp = right[i][j+1];
                if(grid[i][j] == 'E')
                    right[i][j] = tmp + 1;
                else
                    right[i][j] = tmp;
            }
        }
        for(int j = 0;j < n;j++){
            for(int i = 0;i < m;i++){
                int tmp;
                if(i == 0 || grid[i][j] == 'W')
                    tmp = 0;
                else
                    tmp = top[i-1][j];
                if(grid[i][j] == 'E')
                    top[i][j] = tmp + 1;
                else
                    top[i][j] = tmp;
            }
        }
        for(int j = 0;j < n;j++){
            for(int i = m-1;i >= 0;i--){
                int tmp;
                if(i == m-1 || grid[i][j] == 'W')
                    tmp = 0;
                else
                    tmp = down[i+1][j];
                if(grid[i][j] == 'E')
                    down[i][j] = tmp + 1;
                else
                    down[i][j] = tmp;
            }
        }
        int max_num = 0;
        for(int i = 0;i < m;i++){
            for(int j = 0;j < n;j++){
                if(grid[i][j] == '0')
                    max_num = max(max_num,left[i][j] + right[i][j] + top[i][j] + down[i][j]);
            }
        }
        return max_num;
    }
};