First Position of Target
ID: 14; easy; 二分查找
Solution 1 (Java)
public class Solution {
/**
* @param nums: The integer array.
* @param target: Target to find.
* @return: The first position of target. Position starts from 0.
*/
public int binarySearch(int[] nums, int target) {
if (nums == null || nums.length == 0) {
return -1;
}
int left = 0;
int right = nums.length - 1;
while (left + 1 < right) {
int mid = left + (right - left) / 2;
if (nums[mid] < target) {
left = mid;
} else {
right = mid;
}
}
if (nums[left] == target) return left;
if (nums[right] == target) return right;
return -1;
}
}
Notes
This is very similar to the classical binary search. However, we do not return mid when we immediately find a value because we are not sure if it is the first position. So, we only reduce the range. Eventually we have the range
[left, right]
that contains the target and we returnleft
if possible to ensure that it is the first position of target.
Last updated
Was this helpful?