153. Find Minimum in Rotated Sorted Array

Description

Suppose an array sorted in ascending order is rotated at some pivot unknown to you beforehand.

(i.e., [0,1,2,4,5,6,7] might become [4,5,6,7,0,1,2]).

Find the minimum element.

You may assume no duplicate exists in the array.

Example 1:

Input: [3,4,5,1,2] 
Output: 1

Example 2:

Input: [4,5,6,7,0,1,2]
Output: 0

Idea

Just a simple dichotomy.

Solution

class Solution {
public:
    int findMin(vector<int>& nums) {
        int length = nums.size();
        if (length < 0) {
            return -1;
        } else if (length == 1) {
            return nums[0];
        } else if (length == 2) {
            return min(nums[0], nums[1]);
        }
        
        int left = 0, right = length - 1, mid = (left + right) / 2;
        while(left < right) {
            if (nums[left] <= nums[mid] && nums[right] < nums[mid]) {
                // means min locates at (mid, right]
                left = mid + 1;
                mid = ((mid + 1) + right) / 2;
            } else if (nums[right] >= nums[mid]) {
                // means min locates at [left, mid]
                right = mid;
                mid = (left + mid) / 2;
            }
        }
        return nums[left];
    }
};