[LeetCode 153] Find minimum in rotated sorted array (medium)

Solution 1

1. Given it is a rotated array, then the left first half values are greater than the right later half values.
2. Purpose: To find the last position of the 1st half and first position of 2nd half
3. In order to do this, we need to keep left pointer always points to 1st half, and right pointer always points to 2nd half
4. when distance between left and right pointer == 1, then we found the last position of the 1st half and first position of 2nd half; then the minimum one is the first position of the 2nd half

Solution 2

1. 求出middle，用nums [middle]与nums[end]比较

if (nums [middle] < nums[end]) {
   end = middle;
} else if (nums [middle] > nums[end]) {
   start = middle;
}

2. 当 start + 1 == end时，跳出循环。return Math.min (nums [start], nums[end]);

Code1

// iterative solution
class Solution {
    public int findMin(int[] nums) {
        int left = 0;
        int right = nums.length - 1;
        
        // purpose: to find the last element of subarray1 and first element of subarray2
        // In order to do this, we need to keep left pointer always points to subarray1, and right poitner --> subarray2
        // when distance between left and right is 1, then we found the last of subarray 1 and first of subarray2
        // minimum is the first element of subarray2
        
        // this condition guarantee that it is rotated, if not then the first element is the mini
        while (nums[left] > nums[right]) {
            int middle = left + (right - left) / 2;
            
            if (left + 1 == right) {
                return nums[right];
            }
            
            if (nums[middle] > nums[left]) {
                // middle is inside subarray1, so move left to middle
                left = middle;
            } else {
                // middle is inside subarray2, then move right to middle
                right = middle;
            }
        }
        
        return nums[left];
    }
}


// recursion solution
class Solution {
    public int findMin(int[] nums) {
        int left = 0;
        int right = nums.length - 1;
        
        return findMin (nums, left, right);
    }
    
    public int findMin (int[] nums, int left, int right) {
        if (nums[left] < nums[right]) {
            return nums[left];
        }
        
        if (left + 1 >= right) {
            return Math.min (nums[left], nums[right]);
        }
        
        
        int middle = left + (right - left) / 2;
        int min1 = findMin (nums, left, middle);
        int min2 = findMin (nums, middle + 1, right);

        return Math.min (min1, min2);
        
    }

}

Code2

class Solution {
    public int findMin(int[] nums) {
        if (nums == null || nums.length == 0)
            return 0;
        
        int start = 0;
        int end = nums.length - 1;
        
        while (start + 1 < end) {
            int middle = start + (end - start) / 2;
            
            if (nums [middle] < nums[end]) {
                end = middle;
            } else if (nums [middle] > nums[end]) {
                start = middle;
            }
        }
        
        return Math.min (nums [start], nums[end]);
    }
}