力扣，https://leetcode.cn/problems/que-shi-de-shu-zi-lcof/description/

solution1

线性扫描，一般情况

class Solution {public int takeAttendance(int[] records) {boolean[] visited = new boolean[records.length + 1];Arrays.fill(visited, false);for (int i : records) {visited[i] = true;}for (int i = 0; i < visited.length; ++i) {if (!visited[i]) {return i;}}return -1;}
}

solution2

二分搜索

class Solution {public int takeAttendance(int[] records) {return binarySearch(0, records.length - 1, records);}public int binarySearch(int left, int right, int[] records) {while (left <= right) {int mid = left + (right - left) / 2;if (records[mid] == mid) {left = mid + 1;} else {right = mid - 1;}}return left;// 返回right + 1也可}
}