我想这行会有个问题：

return (int)((res+numSubseq(nums,target,i++))%M);

不过，我们可以更容易地解决这个问题，类似地使用排序，然后使用两个指针。

用b.java测试：

import java.util.*;

class Solution {
    private static final int MOD = (int)1e9 + 7;
    public static final int numSubseq(
        final int[] nums,
        final int target
    ) {
        Arrays.sort(nums);
        int[] pows = new int[nums.length];
        pows[0] = 1;

        int subsequences = 0;
        int left = 0;
        int right = nums.length - 1;

        for (int index = 1 ; index < nums.length ; ++index) {
            pows[index] = pows[index - 1] * 2;
            pows[index] %= MOD;
        }

        while (left <= right) {
            if (nums[left] + nums[right] > target) {
                --right;

            } else {
                subsequences += pows[right - left++];
                subsequences %=  MOD;
            }
        }

        return subsequences;
    }
}

class b {
    public static void main(String[] args) {
        System.out.println(new Solution().numSubseq(new int[] {3, 5, 6, 7}, 9));
        System.out.println(new Solution().numSubseq(new int[] {3, 3, 6, 8}, 10));
        System.out.println(new Solution().numSubseq(new int[] {2, 3, 3, 4, 6, 7}, 12));
        System.out.println(new Solution().numSubseq(new int[] {5, 2, 4, 1, 7, 6, 8}, 16));
    }
}

印刷品

4
6
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127