我们需要注意退出while循环的条件是只剩一个元素，但是退出循环以后并不能直接返回n - left的长度，因为我们还要考虑输入数组为[0]的情况，因此必须做一个检查，万一唯一的元素也不符合，就返回0.
class Solution {
public int hIndex(int[] citations) {
/**
* Given a sorted list citations of size n in ascending order,
* one is asked to find the first number citations[i] which meets
* the constraint of citations[i] >= n - i.
*/
if(citations.length == 0){
return 0;
}
int n = citations.length;
int l = 0, r = n - 1;
while(l < r){
int m = l + (r - l) / 2;
if(citations[m] == n - m){
return n - m;
}else if(citations[m] < n - m){
l = m + 1;
}else{
r = m;
}
}
// Post-processing when only one element is left
if(citations[l] >= n - l){
return n - l;
}else{
return 0;
}
}
}