33. Search in Rotated Sorted Array/搜索旋转排序数组

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]).

You are given a target value to search. If found in the array return its index, otherwise return -1.

You may assume no duplicate exists in the array.

Your algorithm's runtime complexity must be in the order of O(log n).

Example 1:

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

Example 2:

Input: nums = [4,5,6,7,0,1,2], target = 3
Output: -1

AC代码

class Solution {
public:
    int search(vector& nums, int target) {
        if (nums.empty()) return -1;
        int ans = -1, left = 0, right = nums.size() - 1, mid;
        while (left <= right) {
            mid = (left + right) / 2;
            if (nums[mid] == target)
                return mid;
            else if (nums[mid] >= nums[left]) {  // mid左边有序
                if (target >= nums[left] && target < nums[mid])
                    right = mid - 1;
                else if (target >= nums[left] && target > nums[mid])
                    left = mid + 1;
                else //target不在左半边
                    left = mid + 1;
            }
            else if (nums[mid] <= nums[right]) {  // mid右边有序
                if (target >= nums[mid] && target <= nums[right])
                    left = mid + 1;
                else //要么target不在右半边，要么在右半边同时target

AC代码（稍稍精简）

class Solution {
public:
    int search(vector& nums, int target) {
        if (nums.empty()) return -1;
        int ans = -1, left = 0, right = nums.size() - 1, mid;
        while (left <= right) {
            mid = (left + right) / 2;
            if (nums[mid] == target) return mid;
            else if (nums[mid] >= nums[left]) {  // mid左边有序
                if (target >= nums[left] && target < nums[mid]) right = mid - 1;
                else left = mid + 1;
            }
            else {  // mid右边有序
                if (target >= nums[mid] && target <= nums[right]) left = mid + 1;
                else right = mid - 1;
            }
        }
        return ans;
    }
};

总结

1、两分钟O(n)，两小时O(logn)，边界问题折磨的要死，后来发现不需要单独考虑1个元素与2个元素的情况
2、二分法过于不熟悉，边界值全靠试错，需多加练习