This program illustrates the use of the STL set_intersection() algorithm
(extended version) to find the values that are in a first vector of integers
but not in a second vector of integers, as well as those integers that are
in the second vector but not in the first, and write them out to a third
vector of integers. In this case the vectors are ordered in the sense that
one integer comes before another if and only if it has a smaller digit sum.
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Here are the values in the vector v1:
11 12 12 12 12 13 14 15
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Here are the values in the vector v2:
2 21 21 31 31 61 71 81
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Here are the values in the vector v3:
101 102 103 104 105 106 107 108 109 110 111 112 113 114 115
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Now we find the values that are in v1 but not in v2, as well as those that are
in v2 but not in v1, and write them out to v3, starting at the beginning of v3.
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Here are the revised contents of v3:
12 12 31 14 15 61 71 81 109 110 111 112 113 114 115
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The iterator returned by the algorithm is pointing at the value 109.
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