【LeetCode每天一题】Set Matrix Zeroes(设置0矩阵)
Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in-place.
Example 1:
Input:
[
[1,1,1],
[1,0,1],
[1,1,1]
]
Output:
[
[1,0,1],
[0,0,0],
[1,0,1]
]
Example 2:
Input:
[
[0,1,2,0],
[3,4,5,2],
[1,3,1,5]
]
Output:
[
[0,0,0,0],
[0,4,5,0],
[0,3,1,0]
]
Follow up:
- A straight forward solution using O(mn) space is probably a bad idea.
- A simple improvement uses O(m + n) space, but still not the best solution.
- Could you devise a constant space solution?
这道题最简单的就是使用O(m*n)的空间来解决问题,但是空间浪费太高。我们可以先对矩阵进行遍历,将0出现的位置的行坐标和列坐标分别记录下来。然后当遍历完毕之后我们根据行坐标和列坐标来设置0.得到最后的结果。时间复杂度为O(m*n), 空间复杂度为O(m+n)。 另外在题中,提到使用O(1)的空间也可以解决问题,我没能想出来,看了参考答案之后明白了他在遇到元素0的时候,先将该元素所对应的行的第一个元素和对应列的第一个元素设置为0,然后遍历结束之后判断首行元素的的值状态将剩余的部分设置为0.最后得到结果。
class Solution(object):
def setZeroes(self, matrix):
"""
:type matrix: List[List[int]]
:rtype: None Do not return anything, modify matrix in-place instead.
"""
if not matrix:
return matrix
row, cloum = len(matrix), len(matrix[0])
tem_row, tem_cloum = set(), set()
for i in range(row): #遍历矩阵得到0的行坐标和列左边
for j in range(cloum):
if matrix[i][j] == 0:
tem_row.add(i)
tem_cloum.add(j)
self.set_zero(matrix, tem_row, tem_cloum) # 进行0设置 def set_zero(self, matrix, tem_row, tem_cloum):
for row in tem_row: # 设置列
for i in range(len(matrix[0])):
if matrix[row][i] != 0:
matrix[row][i] = 0
for cloum in tem_cloum: # 设置行
for j in range(len(matrix)):
if matrix[j][cloum] != 0:
matrix[j][cloum] = 0
常量空间的解法
详细解释链接:https://leetcode.com/problems/set-matrix-zeroes/solution/
class Solution(object):
def setZeroes(self, matrix):
"""
:type matrix: List[List[int]]
:rtype: void Do not return anything, modify matrix in-place instead.
"""
is_col = False
R = len(matrix)
C = len(matrix[0])
for i in range(R): # 遍历矩阵,得到元素为0的地址,并设置该列和该行起始位置为0,
if matrix[i][0] == 0:
is_col = True
for j in range(1, C):
if matrix[i][j] == 0:
matrix[0][j] = 0
matrix[i][0] = 0 for i in range(1, R): # 根据行的开头和列的开头来设置剩余部分的元素值
for j in range(1, C):
if not matrix[i][0] or not matrix[0][j]:
matrix[i][j] = 0 if matrix[0][0] == 0: # 判断起始位置是否为0
for j in range(C):
matrix[0][j] = 0 if is_col: # 第一列全为0
for i in range(R):
matrix[i][0] = 0
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