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() == 0) {
            return -1;
        }
        
        int start = 0;
        int end = array.size() - 1;
        
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            
            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 -1;
    }
};

解法二：

class Solution {
public:

    int find(vector<int> &array, int start, int end, int target) {
        if (start > end) {
            return -1;
        }
        
        int mid = start + (end - start) / 2;
        
        if (array[mid] == target) {

            if (array[mid - 1] != target) {
                return mid;
            }

            return find(array, start, mid - 1, target);
        }
        else if (array[mid] > target) {
            return find(array, start, mid - 1, target);
        }
        else if (array[mid] < target) {
            return find(array, mid + 1, 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 = 0;
        int end = array.size();
        
        return find(array, start, end, target);
        
    }
};