二分查找 && 三分查找

LeetCode34. Find First and Last Position of Element in Sorted Array

题意：找出指定元素出现的范围，Ologn

思路：两次二分

class Solution {
public:
    vector<int> searchRange(vector<int>& nums, int target) {
        int l = 0, r = nums.size() - 1;
        int lx = -1, rx = -1;
        while (l <= r) {
            int mid = (l + r) >> 1;
            if (nums[mid] < target) {
                l = mid + 1;
            } else if (nums[mid] > target) {
                r = mid - 1;
            } else {
                lx = mid;
                r = mid - 1;
            }
        }
        l = 0, r = nums.size() - 1;
        while (l <= r) {
            int mid = (l + r) >> 1;
            if (nums[mid] < target) {
                l = mid + 1;
            } else if (nums[mid] > target) {
                r = mid - 1;
            } else {
                rx = mid;
                l = mid + 1;
            }
        }
        vector<int> res;
        res.push_back(lx);
        res.push_back(rx);
        return res;
    }
};

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