public int missingNumber(int[] nums) {

    //去除特殊情况
    int left = 0 ;
    int right = nums.length-1;
    //为了保证每个数字都经过遍历，所以 <= 最终的结果肯定是存在的，也就是left > right的时候
    while (left <= right) {
        int mid = (left + right) / 2 ;
        if (mid == nums[mid]){
            left = mid +1 ;
        }else {
            // 永远不会出现mid > nums[mid]的情况,下面表示的是 mid < nums[mid]
            right = mid-1;
        }
    }
    return left;
}

@Test
public void test(){
    int[] nums = {0,1} ;
    //9 个数字 mid = 4 相等4 则 left = mid +1
    //9 个数字 mid = 4 小于5 则 right = mid
    //不存在大于

    // int[] nums = {0,1,2,3,4,5,6,7,8} ;
    System.out.println(missingNumber(nums));
}