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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:**

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?

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