275. H-Index II 

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class Solution {
    public int hIndex(int[] citations) {
        int n = citations.length;
        int left = 0, right = n - 1;

        int lastIndex = n;
        while (left <= right) {
            int mid = (left + right) >>> 1;

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

        return n - lastIndex;
    }
}

Binary Search

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class Solution {
    public int hIndex(int[] citations) {
        int n = citations.length;
        int left = 0, right = n - 1;

        while (left <= right) {
            int mid = (left + right) >>> 1;

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

        // now: right, left
        return n - left; // left 为最后一个满足条件的索引
    }
}

References

275. H-Index II