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POJ 1850/ 1496 组合数学

题目地址: 
POJ 1496 Word Index 
POJ 1850 Code

题意： 
1. 每个词是自增的 
2. 同样长度的词是按字典序排练的 
3. 我们把这些词标序 
求某个词的序号

分析： 
组合数学... 
推出公式然后用杨辉三角打表出组合数，具体看http://blog.csdn.net/lyy289065406/article/details/6648492。 
我还是太弱Orz...

代码：

1496：

/*
*  Author:      illuz <iilluzen[at]gmail.com>
*  File:        1496.cpp
*  Create Date: 2014-05-24 23:38:54
*  Descripton:   
*/

#include <cstdio>
#include <cstring>
#include <cmath>

const int N = 27;

int c[N][N], len, sum;
char str[11];

void init() {
	for (int i = 0; i < N; i++)
		for (int j = 0; j <= i; j++)
			if (!j || i == j)
				c[i][j] = 1;
			else
				c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
	c[0][0] = 0;
}

int main() {
	init();

	while (~scanf("%s", str)) {
		len = strlen(str);
		bool flag = true;
		for (int i = 1; i < len; i++)
			if (str[i - 1] >= str[i]) {
				puts("0");
				flag = false;
			}
		if (!flag)
			continue;

		sum = 1;
		for (int i = 1; i < len; i++) {
			sum += c[26][i];
		}

		for (int i = 0; i < len; i++) {
			char ch = (0 == i) ? 'a' : str[i - 1] + 1;
			while (ch <= str[i] - 1) {
				sum += c['z' - ch][len - i - 1];
				ch++;
			}
		}

		printf("%d\n", sum);
	}
	return 0;
}

1850：

/*
*  Author:      illuz <iilluzen[at]gmail.com>
*  File:        1850.cpp
*  Create Date: 2014-05-24 23:10:11
*  Descripton:   
*/

#include <cstdio>
#include <cstring>
#include <cmath>

const int N = 27;

int c[N][N], len, sum;
char str[11];

void init() {
	for (int i = 0; i < N; i++)
		for (int j = 0; j <= i; j++)
			if (!j || i == j)
				c[i][j] = 1;
			else
				c[i][j] = c[i - 1][j - 1] + c[i - 1][j];
	c[0][0] = 0;
}

int main() {
	init();

	scanf("%s", str);
	len = strlen(str);
	for (int i = 1; i < len; i++)
		if (str[i - 1] >= str[i]) {
			puts("0");
			return 0;
		}

	sum = 1;
	for (int i = 1; i < len; i++) {
		sum += c[26][i];
	}

	for (int i = 0; i < len; i++) {
		char ch = (0 == i) ? 'a' : str[i - 1] + 1;
		while (ch <= str[i] - 1) {
			sum += c['z' - ch][len - i - 1];
			ch++;
		}
	}

	printf("%d\n", sum);
	return 0;
}

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POJ 1850/ 1496 组合数学

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原文地址：http://blog.csdn.net/hcbbt/article/details/26883137