NC209784HarmonyPairs题解

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Roundgod is obsessive about numbers. Let S(x)S(x)S(x) be the sum of the digits of xxx in decimal notation, (A,B)(A,B)(A,B) is a harmony pair if and only if S(A)>S(B)S(A)>S(B)S(A)>S(B). Roundgod is given NNN, and she wants to count the number of harmony pairs (A,B)(A,B)(A,B) modulo 109+710^9+7109+7 satisfying 0≤A≤B≤N0le Ale Ble N0≤A≤B≤N.

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