LeetCode 33. Search in Rotated Sorted Array

薛离子

于 2017-02-03 23:48:26 发布

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分类专栏： LeetCode LeetCode 文章标签：有序翻转数组查找 rotated sorted leetcode 33

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本文介绍了一种在未知旋转点的升序数组中使用二分查找法寻找目标值的方法。该算法能够有效判断目标值位于数组的哪一半，并相应调整搜索范围。

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描述
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.

分析
二分查找，难度主要在于左右边界的确定。

代码

class Solution {
public:
    int search(vector<int>& nums, int target) {
        int first = 0;
        int last = nums.size();

        while (first != last) {
            int mid = (first + last) / 2;

            if (nums[mid] == target)
                return mid;

            if (nums[first] <= nums[mid]) {
                if (nums[first] <= target && target < nums[mid])
                    last = mid;
                else
                    first = mid + 1;
            }
            else {
                if (nums[mid] < target && target <= nums[last - 1])
                    first = mid + 1;
                else 
                    last = mid;
            }
        }
        return -1;
    }
};