Maximum Number in Mountain Sequence

O(logN)

Important Point:

if there "mid" exists, it has at laest three elements in "nums", because otherwise it will not come inside the loop. So later, we don't have to check whether "mid -1" and "mid + 1" are OutOfBond.

public class Solution {
    /**
     * @param nums a mountain sequence which increase firstly and then decrease
     * @return then mountain top
     */
    public int mountainSequence(int[] nums) {
        // 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] > nums[mid - 1] && nums[mid] > nums[mid + 1]) {
                return nums[mid];
            } else if (nums[mid] > nums[mid - 1] && nums[mid] < nums[mid + 1]) {
                start = mid;
            } else {
                end = mid;
            }
        }
        
        if (nums[start] >= nums[end]) {
            return nums[start];
        }
        
        if (nums[end] > nums[start]) {
            return nums[end];
        }
        
        return -1;
    }
}