14. First Position of Target 【easy】

14. First Position of Target 【easy】

For a given sorted array (ascending order) and a targetnumber, find the first index of this number in O(log n) time complexity.

If the target number does not exist in the array, return -1.

Example

If the array is [1, 2, 3, 3, 4, 5, 10], for given target 3, return 2.

Challenge

If the count of numbers is bigger than 2^32, can your code work properly?

解法一：

 class Solution {
 public:
     /**
      * @param nums: The integer array.
      * @param target: Target number to find.
      * @return: The first position of target. Position starts from 0.
      */
     int binarySearch(vector<int> &array, int target) {
         if (array.size() == ) {
             return -;
         }

         int start = ;
         int end = array.size() - ;

         while (start +  < end) {
             int mid = start + (end - start) / ;

             if (array[mid] == target) {
                 end = mid;
             }
             else if (array[mid] < target) {
                 start = mid;
             }
             else if (array[mid] > target) {
                 end = mid;
             }
         }

         if (array[start] == target) {
             return start;
         }

         if (array[end] = target) {
             return end;
         }

         return -;
     }
 };

解法二：

 class Solution {
 public:

     int find(vector<int> &array, int start, int end, int target) {
         if (start > end) {
             return -;
         }

         int mid = start + (end - start) / ;

         if (array[mid] == target) {

             if (array[mid - ] != target) {
                 return mid;
             }

             return find(array, start, mid - , target);
         }
         else if (array[mid] > target) {
             return find(array, start, mid - , target);
         }
         else if (array[mid] < target) {
             return find(array, mid + , end, target);
         }

     }

     /**
      * @param nums: The integer array.
      * @param target: Target number to find.
      * @return: The first position of target. Position starts from 0.
      */
     int binarySearch(vector<int> &array, int target) {
         // write your code here

         int start = ;
         int end = array.size();

         return find(array, start, end, target);

     }
 };