215. Kth Largest Element in an Array

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example, Given [3,2,1,5,6,4] and k = 2, return 5.

Note: You may assume k is always valid, 1 ? k ? array's length.

Credits: Special thanks to @mithmatt for adding this problem and creating all test cases.

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找出序列中第k大的数。使用快排算法中的partition函数辅助，加速寻找。基于partition的算法时间复杂度为O(n)。

代码：

class Solution { public: int findKthLargest(vector<int>& nums, int k) { --k; int l = 0, r = nums.size() - 1; while(true) { int idx = partition(nums, l, r); if(idx == k) { return nums[idx]; } else if(idx < k) { l = idx+1; } else { r = idx-1; } } } private: int partition(vector<int>& nums, int l, int r) { int pivot = nums[l], left = l; ++l; while(l <= r) { if(nums[l] < pivot && nums[r] > pivot) { swap(nums[l++], nums[r--]); } if(nums[l] >= pivot) ++l; if(nums[r] <= pivot) --r; } swap(nums[left], nums[r]); return r; } };