leetcode- 215. Kth Largest Element in an Array（基于插入排序思想）

原题

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.

代码分析

求无序数组（可能含有重复元素）的第 K 个最大值。提供一种基于插入排序的求解方法。

基于插入排序求Kth最大值

调用自己写的 NthMax类的过程，这个方法相对繁琐一些。

    public int FindKthLargest(int[] nums, int k)     {            NthMax myNth = new NthMax(k);            foreach (var item in nums)            {                myNth.Insert(item);            }            return myNth.Nth;    }

API 实现细节：

        public class NthMax        {            //构造函数            public NthMax(int nth)            {                _nth = nth;            }            //第n个最大值               public int Nth            {                get                {                    int dis = _maxVals.Length-_nth;                    return _maxVals[dis];                }            }            public void Insert(int target)            {                if (_maxVals == null)                {                    _maxVals = new int[1];                    _maxVals[0] = target;                    return;                }                if (_maxVals.Length < _nth)                {                    int insertPos = searchInsertPos(target);                    insertAt(insertPos, target);                }                else if (_maxVals.Length == _nth)                {                    int insertPos = searchInsertPos(target);                    if (insertPos > 0)                    {                        moveBackward(insertPos - 1);                        _maxVals[insertPos - 1] = target; //在空出来的位置处插入目标元素                    }                }            }         

私有方法：

   private int[] _maxVals;            private int _nth;            // 如果target不在_maxVals中，则找到它的插入位置；如果在直接返回-1            private int searchInsertPos(int target)            {                int lo = 0;                int hi = _maxVals.Length;                while (lo < hi)                {                    int mi = (lo + hi) >> 1;                    if (target < _maxVals[mi]) //目标值小于中间位置的数                        hi = mi;                    else  //不小于中间位置的值                        lo = lo + 1;                }                return lo;            }            //在有序数组index处插入目标元素target            private void insertAt(int index, int target)            {                int[] tmp = new int[_maxVals.Length + 1];                for (int i = 0; i < index; i++)                {                    tmp[i] = _maxVals[i];                }                tmp[index] = target;                for (int i = index; i < _maxVals.Length; i++)                {                    tmp[i + 1] = _maxVals[i];                }                _maxVals = tmp;            }            // 从beginIndex开始覆盖性后移1步            private void moveBackward(int beginIndex)            {                if (beginIndex < 1)                    return;                for (int i = 1; i <= beginIndex; i++)                {                    _maxVals[i - 1] = _maxVals[i];                }            }        }