[LintCode] 462 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, return 2.

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

Given [1, 2, 3, 4, 5] and target = 6, return 0.


Challenge
Time complexity in O(logn)

4/26/2017

算法班

找左右边界，第一个边界找不出来时候返回0

public class Solution {
    /**
     * @param A an integer array sorted in ascending order
     * @param target an integer
     * @return an integer
     */
    public int totalOccurrence(int[] A, int target) {
        if (A == null || A.length == 0) return 0;
        int left, right;

        int start = 0, end = A.length - 1;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (A[mid] >= target) {
                end = mid;
            } else {
                start = mid;
            }
        }
        if (A[start] == target) {
            left = start;
        } else if (A[end] == target) {
            left = end;
        } else {
            return 0;
        }

        start = left;
        end = A.length - 1;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (A[mid] <= target) {
                start = mid;
            } else {
                end = mid;
            }
        }

        if (A[end] == target) {
            right = end;
        } else if (A[start] == target) {
            right = start;
        } else {    // shouldn't come to this branch
            return 0;
        }
        return right - left + 1;
    }
}