Give you a list of the length of N. N is big, but you don't know how big N is. Your task is to randomly remove k elements from these n elements.

Give you a list of the length of N. N is big, but you don't know how big N is. Your task is to randomly remove k elements from these n elements. You can only traverse this list once. Your algorithm must ensure that the extracted elements happen to have K, and that they are completely random (with equal probability of occurrence).

The answer is from the network: Reservoir sampling: First save the first k elements, starting from the first k+1 elements, the probability of 1/i (i=k+1, k+2,..., N) selected the first element, and randomly replace a saved record, so that the traversal once to get k elements, can be guaranteed to completely randomly selected.