【1498. 满足条件的子序列数目】二分查找

import java.util.Arrays;

class Solution {
    public int numSubseq(int[] nums, int target) {
        Arrays.sort(nums);
        int len = nums.length;
        long ans = 0l;
        for (int i = 0; i < len; i++) {
            if( nums[i]*2 <= target){
                int idx = upperBound(nums,0,len,target-nums[i]);
                int rightmost = idx - 1;
                ans = ans + pow(rightmost-i);
                ans %= 1000000007;
            }
        }
        return (int)ans;
    }

    public long pow(int x){
        if( x == 0){
            return 1;
        }
        if( x == 1){
            return 2;
        }
        long y = pow(x/2);
        if( x % 2 == 1){
            return y*y*2%1000000007;
        }
        return y*y%1000000007;
    }

    /**
     * 返回大于key的第一个
     *
     * @param arr
     * @param start
     * @param high
     * @param key
     * @return
     */
    public  int upperBound(int[] arr, int start, int high, int key) {
        while (start < high) {
            int mid = (start+high)/2;

            if(arr[mid] >key){
                high = mid;
            }else{
                start = mid+1;
            }
        }
        return high;
    }

}