34.find-first-and-last-position-of-element-in-sorted-array

题目地址

https://leetcode.com/problems/find-first-and-last-position-of-element-in-sorted-array

题目大意

https://leetcode-cn.com/problems/find-first-and-last-position-of-element-in-sorted-array/

解题思路

利用二分查找

1. 对nums进行二分查找到target的位置mid
2. nums[low, mid - 1]内二分查找到第一个target的索引
3. nums[mid + 1, high]内二分查找到最后一个target的索引

C++代码

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        vector<int> pos = {-1, -1};
        int low = 0, high = int(nums.size()) - 1;
        while (low <= high) {
            int mid = (high + low) / 2;
            if (nums[mid] == target) {
                // find start
                pos = {mid, mid};
                int tempLow = low;
                int tempHigh = mid - 1;
                while (tempLow <= tempHigh) {
                    int tempMid = (tempLow + tempHigh) / 2;
                    if (nums[tempMid] == target) {
                        pos[0] = tempMid;
                        tempHigh = tempMid - 1;
                    } else {
                        tempLow = tempMid + 1;
                    }
                }
                // find end
                tempLow = mid + 1;
                tempHigh = high;
                while (tempLow <= tempHigh) {
                    int tempMid = (tempLow + tempHigh) / 2;
                    if (nums[tempMid] == target) {
                        pos[1] = tempMid;
                        tempLow = tempMid + 1;
                    } else {
                        tempHigh= tempMid - 1;
                    }
                }
                break;
            }
            if (nums[mid] < target) {
                low = mid + 1;
            } else {
                high = mid - 1;
            }
        }
        
        return pos;
    }
};