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; } };