解题思路：二分查找

题解一

class Solution {
    public int countTarget(int[] scores, int target) {
        // 搜索右边界 right
        int i = 0, j = scores.length - 1;
        while(i <= j) {
            int m = (i + j) / 2;
            if(scores[m] <= target) i = m + 1;
            else j = m - 1;
        }
        int right = i;
        // 若数组中无 target ，则提前返回
        if(j >= 0 && scores[j] != target) return 0;
        // 搜索左边界 right
        i = 0; j = scores.length - 1;
        while(i <= j) {
            int m = (i + j) / 2;
            if(scores[m] < target) i = m + 1;
            else j = m - 1;
        }
        int left = j;
        return right - left - 1;
    }
}

题解二（优化版）

class Solution {
    public int countTarget(int[] scores, int target) {
        return helper(scores, target) - helper(scores, target - 1);
    }
    int helper(int[] scores, int tar) {
        int i = 0, j = scores.length - 1;
        while(i <= j) {
            int m = (i + j) / 2;
            if(scores[m] <= tar) i = m + 1;
            else j = m - 1;
        }
        return i;
    }
}