【leetcode】1004. Max Consecutive Ones III

题目如下:

Given an array A of 0s and 1s, we may change up to K values from 0 to 1.

Return the length of the longest (contiguous) subarray that contains only 1s.

Example 1:

Input: A = [1,1,1,0,0,0,1,1,1,1,0], K = 2
Output: 6
Explanation:
[1,1,1,0,0,1,1,1,1,1,1]
Bolded numbers were flipped from 0 to 1. The longest subarray is underlined.

Example 2:

Input: A = [0,0,1,1,0,0,1,1,1,0,1,1,0,0,0,1,1,1,1], K = 3
Output: 10
Explanation:
[0,0,1,1,1,1,1,1,1,1,1,1,0,0,0,1,1,1,1]
Bolded numbers were flipped from 0 to 1. The longest subarray is underlined.

Note:

  1. 1 <= A.length <= 20000
  2. 0 <= K <= A.length
  3. A[i] is 0 or 1

解题思路:如果我们把所有0所在位置对应的下标存入一个数组inx_0中,例如Example 2的inx_0 = [0,1,4,5,9,12,13,14],因为最多把K个0变成1,要构造出最长的全为1的连续子数组,那么很显然所有进行反转操作的0所对应下标在inx_0中必须是连续的。接下来开始遍历数组A,在K大于0的条件下,遇到1则count_1加1,遇到0的话则K减1并且count_1加1(表示这个0反转成1);如果在K=0的情况下遇到0,那么需要去掉第一个0变成的1和这个0之前连续的1的数量,即count减1(减去第一个0变成1的计数),再减去第一个0前面连续的1的数量i,记为count_1减i。记录count_1出现的最大值,直到数组A遍历完成为止。

代码如下:

class Solution(object):
def longestOnes(self, A, K):
"""
:type A: List[int]
:type K: int
:rtype: int
"""
count_1 = 0
zero_inx = []
res = 0
zero_before = -1
for i in range(len(A)):
if A[i] == 1:
count_1 += 1
else:
zero_inx.append(i)
if K > 0:
K -= 1
count_1 += 1
elif K == 0:
res = max(res,count_1)
first_0 = zero_inx.pop(0)
count_1 -= (first_0 - zero_before - 1)
zero_before = first_0
res = max(res, count_1)
return res
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