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Possible Duplicate:
Finding three elements in an array whose sum is closest to an given number
How can I write an Objective C code to check if the sum of any three numbers in an array/list matches a given number?
step 1: sort, O(nlgn)
step 2: iterate every number, say A,(this costs O(n)), then check whether the sum of any two numbers equals to the given number minus A(this is a classic problem which costs O(n))
total complexity: O(n^2)