33. Search in Rotated Sorted Array.cpp

//binary search
class Solution {
public:
    int findMinIdx(vector<int>& nums){
        int l = 0, r = nums.size()-1;
        
        while(l <= r){
            int mid = l + (r-l)/2;
            if(nums[mid] > nums.back()){
                //ex: [4,5,6,7,0,1,2], 7 > 2
                //so search in right part
                l = mid+1;
            }else{
                //right part is monotonic,
                //so search in left part
                r = mid-1;
            }
        }
        
        return l;
    };
    
    int search(vector<int>& nums, int target) {
        int n = nums.size();
        if(n == 0) return -1;
        int minIdx = findMinIdx(nums);
        if(nums[minIdx] == target) return minIdx;
        
        // cout << "minIdx: " << minIdx << endl;
        
        //this is the important part!!
        int l, r;
        if(nums[n-1] >= target){
            //we can find target in left part
            l = minIdx + 1;
            r = n-1;
        }else{
            //we can find target in left part
            l = 0;
            r = minIdx-1;
        }
        
        while(l <= r){
            int mid = l + (r-l)/2;
            
            // cout << l << " " << mid << " " << r << endl;
            
            if(nums[mid] == target){
                return mid;
            }else if(nums[mid] > target){
                r = mid-1;
            }else{
                l = mid+1;
            }
        }
        
        return -1;
    }
};