[LintCode] 458 Last Position of Target

Description
Find the last position of a target number in a sorted array. Return -1 if target does not exist.


Example Given [1, 2, 2, 4, 5, 5].
For target = 2, return 2.
For target = 5, return 5.
For target = 6, return -1.


思路
二分查找。
注意在nums[mid] == target的时候，说明找到了一个。我们是要找最后一个target，所以要从这个找到的地方开始往后看，看还有没有。所以start = mid。


Code

public class Solution {
    /**
     * @param nums: An integer array sorted in ascending order
     * @param target: An integer
     * @return an integer
     */
    public int lastPosition(int[] nums, int target) {
        // Write your code here
        
        if (nums == null || nums.length == 0) {
            return -1;
        }
        
        int start = 0;
        int end = nums.length - 1;
        while (start + 1 < end) {
            int mid = start + (end - start) / 2;
            if (nums[mid] > target) {
                end = mid;
            }
            else if (nums[mid] < target) {
                start = mid;
            }
            else {
                start = mid;
            }
        }
        
        if (nums[end] == target) {
            return end;
        }
        if (nums[start] == target) {
            return start;
        }
        return -1;
    }
}