二分查找一＞0~n-1中缺失的数字(点名)

Set<Integer> set = new HashSet<>();
        for(int x : records) set.add(x);

        for(int i = 0; i < set.size(); i++){
        if(!set.contains(i)) return i;
        }
        return set.size();

int ret = 0;
        for (int i = 0; i < records.length; i++) {
            ret ^= records[i];
            ret ^= i + 1;
        }

        return ret;

int ret = 0;
        for(int i = 0; i < records.length; i++){
            if(records[i] != i) return i;
        }
        return records.length;

        int left = 0,right = records.length-1;
        while(left < right){
            int mid = left + (right - left) / 2;
            if(records[mid] == mid) left = mid+1;
            else right = mid;
        }
        
        //特殊情况：整个数组元素及对应下标都一样时
        return records[left] == left ? left+1 : left;