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题目中的示例为价值矩阵
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有3种初始情况，需要分开考虑
class Solution:
def maxValue(self, grid: List[List[int]]) -> int:
for i in range(len(grid)):
for j in range(len(grid[0])):
if i == 0 and j == 0: continue
if i == 0: grid[i][j] += grid[i][j - 1]
elif j == 0: grid[i][j] += grid[i - 1][j]
else: grid[i][j] += max(grid[i][j - 1], grid[i - 1][j])
return grid[-1][-1]
