Medium
Given a m x n grid filled with non-negative numbers, find a path from top left to bottom right which minimizes the sum of all numbers along its path.

Note: You can only move either down or right at any point in time.

简单题，但不会用二位滚动数组

class Solution {
    public int minPathSum(int[][] grid) {
        if (grid == null || grid.length == 0 || grid[0].length == 0){
           return 0;
        }
        int n = grid.length;
        int m = grid[0].length;
        int[][] f = new int[n][m];
        f[0][0] = grid[0][0];
        for (int i = 1; i < m; i++){
            f[0][i] = f[0][i - 1] + grid[0][i];
        }
        for (int i = 1; i < n; i++){
            f[i][0] = f[i - 1][0] + grid[i][0];
        }
        //function
        for (int i = 1; i < n; i++){
            for (int j = 1; j < m; j++){
                f[i][j] = Math.min(f[i][j - 1], f[i - 1][j]) + grid[i][j];
            }
        }
        //answer
        return f[n - 1][m - 1];
    }
}