LeetCode-in-Java
3770. Largest Prime from Consecutive Prime Sum
Medium
You are given an integer n.
Return the largest prime number less than or equal to n that can be expressed as the sum of one or more consecutive prime numbers starting from 2. If no such number exists, return 0.
Example 1:
Input: n = 20
Output: 17
Explanation:
The prime numbers less than or equal to n = 20 which are consecutive prime sums are:
-
2 = 2 -
5 = 2 + 3 -
17 = 2 + 3 + 5 + 7
The largest is 17, so it is the answer.
Example 2:
Input: n = 2
Output: 2
Explanation:
The only consecutive prime sum less than or equal to 2 is 2 itself.
Constraints:
1 <= n <= 5 * 105
Solution
public class Solution {
static int[] ppr = {
2, 5, 17, 41, 197, 281, 7699, 8893, 22039, 24133, 25237, 28697, 32353, 37561, 38921, 43201,
44683, 55837, 61027, 66463, 70241, 86453, 102001, 109147, 116533, 119069, 121631, 129419,
132059, 263171, 287137, 325019, 329401, 333821, 338279, 342761, 360979, 379667, 393961,
398771
};
public int largestPrime(int n) {
int i = 0;
int j = ppr.length - 1;
int r = 0;
while (i <= j) {
int m = (i + j) >> 1;
if (ppr[m] <= n) {
r = ppr[m];
i = m + 1;
} else {
j = m - 1;
}
}
return r;
}
}