Max sum of 3 consecutive elements in [2, 1, 5, 1, 3, 2]
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first window [2, 1, 5]  sum=8  best=8
right=3  +1 -2  window=[1, 5, 1]  sum=7  best=8
right=4  +3 -1  window=[5, 1, 3]  sum=9  best=9
right=5  +2 -5  window=[1, 3, 2]  sum=6  best=9

answer = 9
