zoukankan      html  css  js  c++  java
  • [LeetCode#60]Permutation Sequence

    Problem:

    The set [1,2,3,…,n] contains a total of n! unique permutations.

    By listing and labeling all of the permutations in order,
    We get the following sequence (ie, for n = 3):

    1. "123"
    2. "132"
    3. "213"
    4. "231"
    5. "312"
    6. "321"

    Given n and k, return the kth permutation sequence.

    Note: Given n will be between 1 and 9 inclusive.

    Analysis:

    Once you understand the mathematical principle behind this problem, the idea could be very easy.
    To successfully implement out the solution, you must equip good programming skills.
    Basic idea:
    According to the example, the small number should be placed before large number at first. which means:
    For num1 (num2 ... num9), 
    There should be permutations among range (num2 ... num9), before we change num1. 
    Apparently there are 8! permutations in the range(num2 ... num9). This is the same for every num in the combination. 
    
    By taking advantage of this truth, we could calcualte the index for each position. Suppose k start from 0. 
    index[0] = k/(n - 1)!
    index[1] = k'/(n - 2)! Note: k' = k % (n - 1)
    
    Skill
    1. To use divide and mod operation, we make k start from 0. 
    Benefit: the calculated index could be directly used for locating numbers. 
    k--;
    
    2. Use ArrayList for recording the available nums. (This method is very elegant! By taking the self adjustment property of the ArrayList, we no longer need to record which num was already used or not.) 
    for (int i = 1; i <= n; i++) {
        num.add(i);
    }
    ...
    while (round >= 0) {
        int index = k / factorial;
        k %= factorial;
        ret.append(num.get(index));
        num.remove(index);
        if (round > 0)
            factorial /= round;
        round --; 
    }
    
    Note: 
    index = k / factorial;
    The index must in the range of [0, round] when enter the while. 
    Cause:
    k %= factorial; <factorial is (round+1)>
    the k is in the range of [(round-1)!, (round)!]
    
    3. Get initial factorial, adjust it along the loop. 
    a. get (n-1)!
    for (int i = 2; i <= n - 1; i++) {
        factorial *= i;
    }
    b. adjust it for next round. 
    if (round > 0)
        factorial /= round;
    round --; 
    c. get current index, and prepare k for getting next index
    int index = k / factorial;
    k %= factorial;
    
    This problem is not hard, but the programming skills it requires are not easy. 
    
    Time complexity:
    Apparently, we need to computate the index for n factorial from (n to 1). For each computation, there is a append and remove operation. the remove operation would take O(n), thus the total time complexity is O(n^2).

    Solution:

    public class Solution {
        public String getPermutation(int n, int k) {
            if (n <= 0 || n > 9)
                return "";
            k--;
            StringBuffer ret = new StringBuffer();
            ArrayList<Integer> num = new ArrayList<Integer> ();
            int factorial = 1; 
            for (int i = 1; i <= n; i++) {
                num.add(i);
            }
            for (int i = 2; i <= n - 1; i++) {
                factorial *= i;
            }
            int round = n - 1;
            while (round >= 0) {
                int index = k / factorial;
                k %= factorial;
                ret.append(num.get(index));
                num.remove(index);
                //avoid the case at last round
                if (round > 0)
                    factorial /= round;
                round --; 
            }
            return ret.toString();
        }
    }
  • 相关阅读:
    Webservice实践(七)CXF 与Spring结合+tomcat发布
    slave have equal MySQL Server UUIDs原因及解决
    VM+CentOS+hadoop2.7搭建hadoop完全分布式集群
    Ubuntu下MySQL主从同步配置
    解决Ubuntu14.04下vi编辑器不能使用方向键和退格键问题
    Ubuntu解压缩zip,tar,tar.gz,tar.bz2
    sersync+rsync原理及部署
    mysqldump备份成压缩包
    linux下如何实现mysql数据库每天自动备份定时备份
    ubuntu mysql 数据库备份以及恢复[命令行]
  • 原文地址:https://www.cnblogs.com/airwindow/p/4746741.html
Copyright © 2011-2022 走看看