325.Maximum Size Subarray Sum Equals k

Tags: Medium Hash Table

Links: https://leetcode-cn.com/problems/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.

Note: The sum of the entire nums array is guaranteed to fit within the 32-bit signed integer range.

Example 1:

Input: nums = [1, -1, 5, -2, 3], k = 3
Output: 4 
Explanation: The subarray [1, -1, 5, -2] sums to 3 and is the longest.

Example 2:

Input: nums = [-2, -1, 2, 1], k = 1
Output: 2 
Explanation: The subarray [-1, 2] sums to 1 and is the longest.

Follow Up: Can you do it in O(n) time?

---

class Solution {
public:
    int maxSubArrayLen(vector<int>& nums, int k) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        int n = nums.size(); if (!n) return 0;
        unordered_map<int, vector<int>> um;
        int sum = nums[0];
        um[sum].push_back(0);
        for (int i = 1; i < n; ++i) {
            sum += nums[i];
            um[sum].push_back(i);
        }

        int res = 0;
        if (um.find(k) != um.end()) {
            res = um[k].back() + 1;
        }
        sum = 0;
        for (int i = 0; i < n; ++i) {
            sum += nums[i];
            if (um.find(sum + k) != um.end()) {
                if (um[sum + k].back() > i) {
                    res = max(res, um[sum + k].back() - i);
                }
            }
        }

        return res;
    }
};

这道题和560基本上代码区别只在26行。仍然是利用哈希表去存储前缀和和下标的对应关系。

时间复杂度是$O(n)$。

当然类似的题目还有209，求最小长度，方法大同小异。