Search in a Big Sorted Array

ID: 447; medium; 在大数组中查找

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Solution 1 (Java)

public class Solution {
    /**
     * @param reader: An instance of ArrayReader.
     * @param target: An integer
     * @return: An integer which is the first index of target.
     */
    public int searchBigSortedArray(ArrayReader reader, int target) {
        int left = 0, right = 1;
        while (reader.get(right) < target) {
            right *= 2;
        }

        while (left + 1 < right) {
            int mid = left + (right - left) / 2;
            int midNum = reader.get(mid);
            if (midNum < target) {
                left = mid;
            } else {
                right = mid;
            }
        }

        if (reader.get(left) == target) return left;
        if (reader.get(right) == target) return right;
        return -1;
    }
}

Notes

• We do not know the upper bound of the range, so we double right starting from 1 until the element at right is larger than the target. Thus, we have a valid range containing a target.

• We cannot return mid immediately if we found a target because the problem requires the first position of the target.