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kthSmallest.h
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kthSmallest.h
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//
// kthSmallest.h
// sort
//
// Created by junlongj on 2019/7/19.
// Copyright © 2019 junl. All rights reserved.
//
#ifndef kthSmallest_h
#define kthSmallest_h
/*
如何在o(n)内查找一个无序数组中的第K大元素.
利用快排分区的思想,将数组一分为3,然后根据pivot和kth的位置关系,继续寻找,最终定位到kth.
*/
int _partition(int nums[], int start, int end){
int i=start;
int j=start;
for (; j<end; j++) {
if (nums[j] < nums[end]) {
std::swap(nums[j], nums[i++]);
}
}
std::swap(nums[i], nums[end]);
return i;
}
int kthSmallest(int nums[], int n,int start, int end, int kth){
//数组分为了三部分,nums[start....pivot-1], nums[pivot], nums[pivot+1,end]
//2...5 k=1
int pivot = _partition(nums, start, end);
if (pivot == kth-1) {
//find
return nums[pivot];
}else if (pivot < kth-1){
//说明在右边
return kthSmallest(nums, n, pivot+1, end, kth-pivot-1);
}else{
//说明在左边
return kthSmallest(nums, n, start, pivot-1, kth);
}
}
#endif /* kthSmallest_h */