Total Occurrence of Target

Description

Given a target number and an integer array sorted in ascending order. Find the total number of occurrences of target in the array.

Example

Given[1, 3, 3, 4, 5]and target =3, return2.

Given[2, 2, 3, 4, 6]and target =4, return1.

Given[1, 2, 3, 4, 5]and target =6, return0.

Challenge

Time complexity in O(logn)

Related problems

Search for a Range

Implementation

Link: http://lintcode.com/en/problem/total-occurrence-of-target/

class Solution {
public:
    /**
     * @param A an integer array sorted in ascending order
     * @param target an integer
     * @return an integer
     */
    int totalOccurrence(vector<int>& A, int target) {
        // Write your code here
        if(A.size() == 0) return 0;

        int start = 0, end = A.size() - 1;
        // find the 1st
        while(start + 1 < end) {
            int mid = start + (end - start) / 2;
            if(A[mid] == target) {
                end = mid;
            } else if(A[mid] < target) {
                start = mid;
            } else {
                end = mid;
            }
        }

        int first = -1;
        if(A[start] == target) {
            first = start;
        } else if(A[end] == target) {
            first = end;
        } else {
            return 0;
        }

        // find the last
        start = 0;
        end = A.size() - 1;
        while(start + 1 < end) {
            int mid = start + (end - start) / 2;
            if(A[mid] == target) {
                start = mid;
            } else if(A[mid] < target) {
                start = mid;
            } else {
                end = mid;
            }
        }

        int last = -1;
        if(A[end] == target) {
            last = end;
        } else if(A[start] == target) {
            last = start;
        } else {
            return 0;
        }

        return last - first + 1;
    }
};