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Brutal-force solution is not hard to think about. But linear space input usually indicates O(n) DP solution.
State design: dp[i]: total sum until index i; and then you need some number manipulation to figure out the equation.
https://www.hackerrank.com/challenges/sam-and-substrings/editorial
#include <stdio.h> #include <iostream> #include <algorithm> #include <vector> #include <string> using namespace std; #define MOD 1000000007 size_t calc(string &str) { size_t len = str.length(); vector<size_t> dp(len + 1); dp[0] = str[0] - ‘0‘; for (int i = 1; i < len; i++) { size_t curr = (i + 1) * (str[i] - ‘0‘) % MOD; curr += 10 * dp[i - 1] % MOD; dp[i] = curr; } // Get sum size_t ret = 0; for (int i = 0; i < len; i++) ret = (ret + dp[i]) % MOD; return ret; } int main() { string in; cin >> in; size_t len = in.length(); size_t r = calc(in); cout << r << endl; return 0; }
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原文地址：http://www.cnblogs.com/tonix/p/4648045.html
