two pointers 双针法

顺序表双针
a + b = m

const int maxn =110;
int a[maxn], count = 0, n, m;

int func() {
	int i = 0, j = n-1;
	while(i >= j) {
		if(a[i] + a[j] == m) {
			printf("%d + %d = %d\n", a[i], a[j], m);
			count++;
			i++;
			j--;
		}
		else if (a[i] + a[j] > m) {
			j--;
		}
		else {
			i++;
		}
	}
}

序列合并

// merge a and b into c
void func(int a[], int b[], int c[], int n, int m) {
	int i = 0, j = 0, k = 0;
	while(i < n && j < m) {
		if(a[i] <= b[j]) {
			c[k++] = a[i++];
		}
		else {
			c[k++] = b[j++];
		}
	}
	while(i < n) c[k++] = a[i++];
	while(j < m) c[k++] = b[j++];
	
}

归并排序

// a[] 1~n
// 递归
const int maxn = 110;

void merge(int a[], int l1, int r1, int l2, int r2) {
	 int i = l1, j = l2;
	 int temp[maxn], k = 0;
	 while(i < r1 && j < r2) {
	 	if(a[i] < a[j]) temp[k++] = a[i++];
	 	else temp[k++] = a[j++];
	 } 
	 while(i < r1) temp[k++] = a[i++];
	 while(j < r2)  temp[k++] = a[j++];
	 for(int i = 0; i < k; ++i) {
	 	a[l1 + i] = temp[i];
	 } 
} 
// 非递归
void mergeSort(int a[], int left, int right) {
	if(left < right) {
		int mid = left + (right - left) / 2;
		mergeSort(a, left, mid);
		mergeSort(mid + 1, right);
		merge(a, left, mid, mid + 1, right);
	}
}

void mergeSort(int a[]) {
	for(int step = 2; step/2 <= n; step *= 2) {
		for(int i = 1; i <= n; i += step) {
			int mid = i + step / 2 - 1;
			if(mid + 1 <= n) {	// 右区间仍有元素 
				merge(a, i, mid, mid + 1, min(i + step - 1, n));
			}
		}
	}
}

// 偷懒直接sort 
void mergeSort(int a[]) {
	for(int step = 2; step/2 <= n; step *= 2) {
		for(int i = 1; i <= n; i += step) {
			sort(a + i, a + min(i + step, n + 1)); // sort左闭右开 
		}
	}
}

快速排序

// quick sort
int partition(int a[], int left, int right) {
	int t = a[left];
	while(left < right) {
		while(left < right && a[right] > t) right--;
		a[left] = a[right];
		while(left < right && a[left] <= t) left ++;
		a[right] = a[left];
	}
	a[left] = t;
	return left;
} 

void quickSort(int a[], int left, int right) {
	if(left < right) {
		int pos = position(a, left, right);
		quickSort(a,left, pos - 1);
		quickSort(a, pos + 1, right);
	}
}

// 添加 随机数
#include <cstdio>
#include <cstdlib>
#include <time.h>

int main() {
	srand((unsigned)time(NULL)); // 生成随机数种子
	// ...
	return 0;
}

int randPartition(int a[], int left, int right) {
	int p = (round(1.0 * rand() / RAND_MAX * (right - left) + left));
	swap(a[p], a[left]);
	// 下同partition 
	int t = a[left];
	while(left < right) {
		while(left < right && a[right] > t) right--;
		a[left] = a[right];
		while(left < right && a[left] <= t) left++;
		a[right] = a[left];
	}
	a[left] = t;
	return left;
} 

随机选择算法
求序列中第K大的元素

int randSelection(int a[], int left, int right, int k) {
	if(left == right) return a[left];
	int p = randPartition(a, left, right);
	int rank = p - left + 1;	// p位置元素的排名
	if(rank == p) return a[rank];
	else if (rank < p) {		// p位置元素排名在k前, 从后半部分继续寻找
		return randSelection(a, p + 1, right, k - m);	// 后半部分寻找第K-rank大的元素
	}
	else {
		return randSelection(a, left, p - 1, k);		// 前半部分寻找第K大的元素
	}
}