Search in a Big Sorted Array
ID: 447; medium; 在大数组中查找
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 thetarget
. Thus, we have a valid range containing atarget
.We cannot return
mid
immediately if we found atarget
because the problem requires the first position of thetarget
.
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