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64. 最小路径和
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https://leetcode-cn.com/problems/minimum-path-sum/

给定一个包含非负整数的 m x n 网格，请找出一条从左上角到右下角的路径，使得路径上的数字总和为最小。

说明：每次只能向下或者向右移动一步。

示例::

    输入:
    [
      [1,3,1],
      [1,5,1],
      [4,2,1]
    ]
    输出: 7

解释: 因为路径 1→3→1→1→1 的总和最小。


.. code:: python

    class Solution(object):
        def minPathSum(self, grid):
            """
            :type grid: List[List[int]]
            :rtype: int
            """
            for i in range(len(grid)):
                for j in range(len(grid[0])):
                    if i == j == 0:
                        continue
                    elif i == 0:
                        grid[i][j] = grid[i][j-1] + grid[i][j]
                    elif j == 0:
                        grid[i][j] = grid[i-1][j] + grid[i][j]
                    else:
                        grid[i][j] = min(grid[i-1][j], grid[i][j-1]) + grid[i][j]
            return grid[-1][-1]