1498. Number of Subsequences That Satisfy the Given Sum Condition

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題目敘述

You are given an array of integers nums and an integer target.

Return the number of non-empty subsequences of nums such that the sum of the minimum and maximum element on it is less or equal to target. Since the answer may be too large, return it modulo 10^9 + 7.

Example 1:

Input: nums = [3,5,6,7], target = 9 Output: 4 Explanation: There are 4 subsequences that satisfy the condition. [3] -> Min value + max value <= target (3 + 3 <= 9) [3,5] -> (3 + 5 <= 9) [3,5,6] -> (3 + 6 <= 9) [3,6] -> (3 + 6 <= 9)

Example 2:

Input: nums = [3,3,6,8], target = 10 Output: 6 Explanation: There are 6 subsequences that satisfy the condition. (nums can have repeated numbers). [3] , [3] , [3,3], [3,6] , [3,6] , [3,3,6]

Example 3:

Input: nums = [2,3,3,4,6,7], target = 12 Output: 61 Explanation: There are 63 non-empty subsequences, two of them do not satisfy the condition ([6,7], [7]). Number of valid subsequences (63 - 2 = 61).

解題思路

Solution

import java.util.Arrays;

class Solution {
    public int numSubseq(int[] nums, int target) {
        int n = nums.length;
        int mod = 1000000007;
        int ans = 0;
        Arrays.sort(nums);

        int[] count = new int[n];
        count[0] = 1;
        for (int i = 1; i < n; i++) {
            count[i] = (count[i - 1] * 2) % mod;
        }

        int left = 0, right = n - 1;
        while (left <= right) {
            if (nums[left] + nums[right] <= target) {
                ans = (ans + count[right - left]) % mod;
                left++;
            } else {
                right--;
            }
        }
        
        return ans;
    }
}