题解 | #缺失数字#

二分：

class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 找缺失数字
     * @param a int整型vector 给定的数字串
     * @return int整型
     */
    int solve(vector<int>& a) {
        // write code here
        int length = a.size();
        int left = 0;
        int right = length - 1;
        int mid;

        // 特例：
        // [0,1,2,3,4,5,6,7,8]   输出 9
        if (right == a[right])
            return length;

        while (left <= right) {
            mid = left + (right - left) / 2;

            if (a[mid] == mid) {
                left = mid + 1;
            }
            else if (a[mid] > mid) {
                right = mid - 1;
            }
        }

        return a[left] - 1;
    }
};