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# 思路1 前缀和 + hash

一次遍历搞定

preSum\[i]定义为 nums\[0] + nums\[1] + ... + nums\[i]

那么preSum\[i] - preSum\[j-1]就是 nums\[j] + nums\[j+1] + ... + nums\[i-1] + nums\[i]

如果\[j, i]区间的和是k，那么就有preSum\[i] - preSum\[j-1] == k

-> preSum\[i] - k == preSum\[j-1]