POJ 3187 -- Backward Digit Sums


http://poj.org/problem?id=3187
FJ and his cows enjoy playing a mental game. They write down the numbers from 1 to N (1 <= N <= 10) in a certain order and then sum adjacent numbers to produce a new list with one fewer number. They repeat this until only a single number is left. For example, one instance of the game (when N=4) might go like this:

    3   1   2   4  
      4   3   6  
        7   9  
         16
Behind FJ's back, the cows have started playing a more difficult game, in which they try to determine the starting sequence from only the final total and the number N. Unfortunately, the game is a bit above FJ's mental arithmetic capabilities.

Write a program to help FJ play the game and keep up with the cows.

http://www.hankcs.com/program/cpp/poj-3187-backward-digit-sums-challenge-programming-contest-2nd-edition-exercises-answers.html
杨辉三角前10行:









1

















1

1















1

2

1













1

3

3

1











1

4

6

4

1









1

5

10

10

5

1







1

6

15

20

15

6

1





1

7

21

35

35

21

7

1



1

8

28

56

70

56

28

8

1

1

9

36

84

126

126

84

36

9

1
杨辉三角第n层第k个数记为Ckn那么=n!/[k!(n-k)!]=n * (n – 1)…*(n – k + 1) / k!
对应着下面这段代码
1
2
3
4
5
6
7
8
9
10
int c(int n, int k)
{
    int result = 1;
    for (int i = 0; i < k; ++i)
    {
        result = result * (n - i) / (i + 1);
    }
 
    return result;
}
上面做了一个简化,因为原始的式子里面分子分母的项数相等所有写进一个loop里。
有了Ck那么即使题目中的初始数字不为1,只要乘上这个系数Ckn就行了。
int c(int n, int k)
{
    int result = 1;
    for (int i = 0; i < k; ++i)
    {
        result = result * (n - i) / (i + 1);
    }
 
    return result;
}
int main(int argc, char *argv[])
{
    int N, Sum;
    cin >> N >> Sum;
    int line[16];
    int i = 0;
    for (; i < N; ++i)
    {
        line[i] = i + 1;
    }
    do 
    {
        int result = 0;
        for (i = 0; i < N; ++i)
        {
            result += c(N - 1, i) * line[i];
        }
        if (result == Sum)
        {
            break;
        }
    while (next_permutation(line, line + N));
    copy(line, line + N, ostream_iterator<int>(cout, " "));
    return 0;
}

http://www.cnblogs.com/7hat/p/3601678.html
分析:这题用全排列的方式非常容易做。首先初始化数组为1-N,然后用STL提供的按字典序生成全排列的函数next_permutation即可枚举全排列。对于每一组数,通过计算可以知道它是否能得出已知结果。最先找到的那组数就是字典序最小的值。
15 int N, finalSum;
17 int a[MAX_N][MAX_N];                //保存序列以及计算序列结果
18 
19 int calulate(){
20     //计算序列所得的最后和
21     for(int i = 1; i < N; i ++){
22         for(int j = 0; j < N - i; j ++){
23             a[i][j] = a[i - 1][j] + a[i - 1][j + 1];
24         }
25     }
26     return a[N - 1][0];
27 }
28 
29 void solve(){
30     //初始化序列
31     for(int i = 0; i < N; i ++)
32         a[0][i] = i + 1;
33     //按字典序枚举全排列
34     do{
35         int sum = calulate();
36         if(sum == finalSum){
37             printf("%d", a[0][0]);
38             for(int i = 1; i < N; i ++){
39                 printf(" %d", a[0][i]);
40             }
41             printf("\n");
42             break;
43         }
44     }while(next_permutation(a[0], a[0] + N));
45 
46 }
http://blog.sina.com.cn/s/blog_6635898a0100kko3.html
思路:n比较小,暴搜。先由杨辉三角的性质:sum = 0Cn-1*num[0] + 1Cn-1*num[1] +``+ n-1Cn-1*num[n-1],先求出底边各个数对应的系数值,然后再暴搜枚举num[i]的值。
http://blog.csdn.net/mullerwch/article/details/38392961
  1. bool visit[15];  
  2. int a[15],b[15];  
  3. int N, sum;  
  4.   
  5. bool per(int k){  
  6.     if(k == (N+1)){  
  7.         int i,j;  
  8.         for(i=1; i<=N; ++i)  
  9.             b[i] = a[i];  
  10.         for(i=1; i<N; ++i){  
  11.             for(j=1; j<=N-i; ++j){  
  12.                 b[j] = b[j]+b[j+1];  
  13.             }  
  14.         }  
  15.         if(b[1] == sum){  
  16.             for(i=1; i<=N; ++i)  
  17.                 printf("%d ", a[i]);  
  18.             printf("\n");  
  19.             return true;  
  20.         }  
  21.         return false;  
  22.     }  
  23.   
  24.     int i;  
  25.     for(i=1; i<=N; ++i){  
  26.         if(!visit[i]){  
  27.             visit[i] = true;  
  28.             a[k] = i;  
  29.             if(per(k+1))  
  30.                 return true;  
  31.             visit[i] = false;  
  32.         }  
  33.     }  
  34.     return false;  
  35. }  
  36. int main(){  
  37.     #ifdef LOCAL  
  38.         freopen("1.in","r", stdin);  
  39.     #endif  
  40.   
  41.     while(~scanf("%d%d", &N, &sum)){  
  42.         memset(visit, falsesizeof(visit));  
  43.         per(1);  
  44.     }  
  45.     return 0;  
  46. }  
Read full article from 3187 -- Backward Digit Sums

Labels

LeetCode (1432) GeeksforGeeks (1122) LeetCode - Review (1067) Review (882) Algorithm (668) to-do (609) Classic Algorithm (270) Google Interview (237) Classic Interview (222) Dynamic Programming (220) DP (186) Bit Algorithms (145) POJ (141) Math (137) Tree (132) LeetCode - Phone (129) EPI (122) Cracking Coding Interview (119) DFS (115) Difficult Algorithm (115) Lintcode (115) Different Solutions (110) Smart Algorithm (104) Binary Search (96) BFS (91) HackerRank (90) Binary Tree (86) Hard (79) Two Pointers (78) Stack (76) Company-Facebook (75) BST (72) Graph Algorithm (72) Time Complexity (69) Greedy Algorithm (68) Interval (63) Company - Google (62) Geometry Algorithm (61) Interview Corner (61) LeetCode - Extended (61) Union-Find (60) Trie (58) Advanced Data Structure (56) List (56) Priority Queue (53) Codility (52) ComProGuide (50) LeetCode Hard (50) Matrix (50) Bisection (48) Segment Tree (48) Sliding Window (48) USACO (46) Space Optimization (45) Company-Airbnb (41) Greedy (41) Mathematical Algorithm (41) Tree - Post-Order (41) ACM-ICPC (40) Algorithm Interview (40) Data Structure Design (40) Graph (40) Backtracking (39) Data Structure (39) Jobdu (39) Random (39) Codeforces (38) Knapsack (38) LeetCode - DP (38) Recursive Algorithm (38) String Algorithm (38) TopCoder (38) Sort (37) Introduction to Algorithms (36) Pre-Sort (36) Beauty of Programming (35) Must Known (34) Binary Search Tree (33) Follow Up (33) prismoskills (33) Palindrome (32) Permutation (31) Array (30) Google Code Jam (30) HDU (30) Array O(N) (29) Logic Thinking (29) Monotonic Stack (29) Puzzles (29) Code - Detail (27) Company-Zenefits (27) Microsoft 100 - July (27) Queue (27) Binary Indexed Trees (26) TreeMap (26) to-do-must (26) 1point3acres (25) GeeksQuiz (25) Merge Sort (25) Reverse Thinking (25) hihocoder (25) Company - LinkedIn (24) Hash (24) High Frequency (24) Summary (24) Divide and Conquer (23) Proof (23) Game Theory (22) Topological Sort (22) Lintcode - Review (21) Tree - Modification (21) Algorithm Game (20) CareerCup (20) Company - Twitter (20) DFS + Review (20) DP - Relation (20) Brain Teaser (19) DP - Tree (19) Left and Right Array (19) O(N) (19) Sweep Line (19) UVA (19) DP - Bit Masking (18) LeetCode - Thinking (18) KMP (17) LeetCode - TODO (17) Probabilities (17) Simulation (17) String Search (17) Codercareer (16) Company-Uber (16) Iterator (16) Number (16) O(1) Space (16) Shortest Path (16) itint5 (16) DFS+Cache (15) Dijkstra (15) Euclidean GCD (15) Heap (15) LeetCode - Hard (15) Majority (15) Number Theory (15) Rolling Hash (15) Tree Traversal (15) Brute Force (14) Bucket Sort (14) DP - Knapsack (14) DP - Probability (14) Difficult (14) Fast Power Algorithm (14) Pattern (14) Prefix Sum (14) TreeSet (14) Algorithm Videos (13) Amazon Interview (13) Basic Algorithm (13) Codechef (13) Combination (13) Computational Geometry (13) DP - Digit (13) LCA (13) LeetCode - DFS (13) Linked List (13) Long Increasing Sequence(LIS) (13) Math-Divisible (13) Reservoir Sampling (13) mitbbs (13) Algorithm - How To (12) Company - Microsoft (12) DP - Interval (12) DP - Multiple Relation (12) DP - Relation Optimization (12) LeetCode - Classic (12) Level Order Traversal (12) Prime (12) Pruning (12) Reconstruct Tree (12) Thinking (12) X Sum (12) AOJ (11) Bit Mask (11) Company-Snapchat (11) DP - Space Optimization (11) Dequeue (11) Graph DFS (11) MinMax (11) Miscs (11) Princeton (11) Quick Sort (11) Stack - Tree (11) 尺取法 (11) 挑战程序设计竞赛 (11) Coin Change (10) DFS+Backtracking (10) Facebook Hacker Cup (10) Fast Slow Pointers (10) HackerRank Easy (10) Interval Tree (10) Limited Range (10) Matrix - Traverse (10) Monotone Queue (10) SPOJ (10) Starting Point (10) States (10) Stock (10) Theory (10) Tutorialhorizon (10) Kadane - Extended (9) Mathblog (9) Max-Min Flow (9) Maze (9) Median (9) O(32N) (9) Quick Select (9) Stack Overflow (9) System Design (9) Tree - Conversion (9) Use XOR (9) Book Notes (8) Company-Amazon (8) DFS+BFS (8) DP - States (8) Expression (8) Longest Common Subsequence(LCS) (8) One Pass (8) Quadtrees (8) Traversal Once (8) Trie - Suffix (8) 穷竭搜索 (8) Algorithm Problem List (7) All Sub (7) Catalan Number (7) Cycle (7) DP - Cases (7) Facebook Interview (7) Fibonacci Numbers (7) Flood fill (7) Game Nim (7) Graph BFS (7) HackerRank Difficult (7) Hackerearth (7) Inversion (7) Kadane’s Algorithm (7) Manacher (7) Morris Traversal (7) Multiple Data Structures (7) Normalized Key (7) O(XN) (7) Radix Sort (7) Recursion (7) Sampling (7) Suffix Array (7) Tech-Queries (7) Tree - Serialization (7) Tree DP (7) Trie - Bit (7) 蓝桥杯 (7) Algorithm - Brain Teaser (6) BFS - Priority Queue (6) BFS - Unusual (6) Classic Data Structure Impl (6) DP - 2D (6) DP - Monotone Queue (6) DP - Unusual (6) DP-Space Optimization (6) Dutch Flag (6) How To (6) Interviewstreet (6) Knapsack - MultiplePack (6) Local MinMax (6) MST (6) Minimum Spanning Tree (6) Number - Reach (6) Parentheses (6) Pre-Sum (6) Probability (6) Programming Pearls (6) Rabin-Karp (6) Reverse (6) Scan from right (6) Schedule (6) Stream (6) Subset Sum (6) TSP (6) Xpost (6) n00tc0d3r (6) reddit (6) AI (5) Abbreviation (5) Anagram (5) Art Of Programming-July (5) Assumption (5) Bellman Ford (5) Big Data (5) Code - Solid (5) Code Kata (5) Codility-lessons (5) Coding (5) Company - WMware (5) Convex Hull (5) Crazyforcode (5) DFS - Multiple (5) DFS+DP (5) DP - Multi-Dimension (5) DP-Multiple Relation (5) Eulerian Cycle (5) Graph - Unusual (5) Graph Cycle (5) Hash Strategy (5) Immutability (5) Java (5) LogN (5) Manhattan Distance (5) Matrix Chain Multiplication (5) N Queens (5) Pre-Sort: Index (5) Quick Partition (5) Quora (5) Randomized Algorithms (5) Resources (5) Robot (5) SPFA(Shortest Path Faster Algorithm) (5) Shuffle (5) Sieve of Eratosthenes (5) Strongly Connected Components (5) Subarray Sum (5) Sudoku (5) Suffix Tree (5) Swap (5) Threaded (5) Tree - Creation (5) Warshall Floyd (5) Word Search (5) jiuzhang (5)

Popular Posts