Leetcode 325. Maximum Size Subarray Sum Equals k
Given an array nums and a target value k, find the maximum length of a subarray that sums to k. If there isn't one, return 0 instead.
Example 1:
Given nums = [1, -1, 5, -2, 3]
, k = 3
,
return 4
. (because the subarray [1, -1, 5, -2]
sums to 3 and is the longest)
Example 2:
Given nums = [-2, -1, 2, 1]
, k = 1
,
return 2
. (because the subarray [-1, 2]
sums to 1 and is the longest)
Follow Up:
Can you do it in O(n) time?
Difficulty:Medium
Category:
Analyze
这道题给我们一个一维数组nums,让我们求和为k最大子数组,默认子数组必须连续,题目中提醒我们必须要在O(n)的时间复杂度完成.
Solution
该方案来自于博客文章:Maximum Size Subarray Sum Equals k 最大子数组之和为k
class Solution {
public:
int maxSubArrayLen(vector<int>& nums, int k) {
// sum: 累加
int sum = 0, res = 0;
unordered_map<int, int> m; // [value -> lastindex]
for (int i = 0; i < nums.size(); ++i) {
sum += nums[i];
//边累加边处理
if (sum == k)
res = i + 1;
else if (m.count(sum - k))
res = max(res, i - m[sum - k]);
// Save the first index when we get the sum
if (!m.count(sum)) m[sum] = i;
}
return res;
}
};