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1. leetcode 75.颜色分类

1.1 题目

题目链接

1.2 思路

1.3 代码

class Solution {
public:void sortColors(vector<int>& nums) {int n = nums.size();for(int i = 0, left = -1, right = n; i < right;){if(nums[i] == 0) swap(nums[++left], nums[i++]);else if(nums[i] == 1) i++;else swap(nums[--right], nums[i]);}}
};

2. leetcode 912.排序数组

2.1 题目

题目链接

2.2 思路

2.3 代码

class Solution {
public:vector<int> sortArray(vector<int>& nums) {srand(time(NULL)); // 种下一个随机数种子qsort(nums, 0, nums.size() - 1);return nums;}// 快排void qsort(vector<int>& nums, int l, int r){if(l >= r) return;int key = getRandom(nums, l, r);int i = l, left = l - 1, right = r + 1;while(i < right){if(nums[i] < key) swap(nums[++left], nums[i++]);else if(nums[i] == key) i++;else swap(nums[--right], nums[i]); }qsort(nums, l, left);qsort(nums, right, r);}int getRandom(vector<int>& nums, int left,int right){int r = rand();return nums[r % (right - left + 1) + left];}
};

3.leetcode 215.数组中的第k个最大元素

3.1 题目

题目链接

3.2 思路

3.3 代码

class Solution {
public:int findKthLargest(vector<int>& nums, int k) {srand(time(NULL));return qsort(nums, 0, nums.size() - 1, k);  }// 1.qsort快排int qsort(vector<int>& nums, int l, int r, int k){// 1.1 随机选择基准元素keyint key = getRandom(nums, l, r);// 1.2 划分范围int left = l - 1, right = r + 1;for(int i = l; i < right;){if(nums[i] < key) swap(nums[i++], nums[++left]);else if(nums[i] == key) i++;else swap(nums[i], nums[--right]);}// 1.3 分情况讨论// 现在数组分为了三段 [l, left] [left+1, right-1] [right, r]if(r - right + 1 >= k) return qsort(nums, right, r, k);else if(r - left >= k) return key;else return qsort(nums, l, left, k - r + left);}// 2.getRandom函数 用于生成随机数int getRandom(vector<int>& nums, int left, int right){return nums[rand() % (right - left + 1) + left];}
};