zoukankan      html  css  js  c++  java
  • [LeetCode] Find Permutation 找全排列

    By now, you are given a secret signature consisting of character 'D' and 'I'. 'D' represents a decreasing relationship between two numbers, 'I' represents an increasing relationship between two numbers. And our secret signaturewas constructed by a special integer array, which contains uniquely all the different number from 1 to n (n is the length of the secret signature plus 1). For example, the secret signature "DI" can be constructed by array [2,1,3] or [3,1,2], but won't be constructed by array [3,2,4] or [2,1,3,4], which are both illegal constructing special string that can't represent the "DI" secret signature.

    On the other hand, now your job is to find the lexicographically smallest permutation of [1, 2, ... n] could refer to the given secret signature in the input.

    Example 1:

    Input: "I"
    Output: [1,2]
    Explanation: [1,2] is the only legal initial spectial string can construct secret signature "I", where the number 1 and 2 construct an increasing relationship.
    

    Example 2:

    Input: "DI"
    Output: [2,1,3]
    Explanation: Both [2,1,3] and [3,1,2] can construct the secret signature "DI", 
    but since we want to find the one with the smallest lexicographical permutation, you need to output [2,1,3]

    Note:

    • The input string will only contain the character 'D' and 'I'.
    • The length of input string is a positive integer and will not exceed 10,000

    这道题给了我们一个由D和I两个字符组成的字符串,分别表示对应位置的升序和降序,要我们根据这个字符串生成对应的数字字符串。由于受名字中的permutation的影响,感觉做法应该是找出所有的全排列然后逐个数字验证,这种方法十有八九无法通过OJ。其实这题用贪婪算法最为简单,我们来看一个例子:

    D D I I D I

    1 2 3 4 5 6 7

    3 2 1 4 6 5 7

    我们不难看出,只有D对应的位置附近的数字才需要变换,而且变换方法就是倒置一下字符串,我们要做的就是通过D的位置来确定需要倒置的子字符串的起始位置和长度即可。通过观察,我们需要记录D的起始位置i,还有D的连续个数k,那么我们只需要在数组中倒置[i, i+k]之间的数字即可,根据上述思路可以写出代码如下:

    解法一:

    class Solution {
    public:
        vector<int> findPermutation(string s) {
            int n = s.size();
            vector<int> res(n + 1);
            for (int i = 0; i < n + 1; ++i) res[i] = i + 1;
            for (int i = 0; i < n; ++i) {
                if (s[i] != 'D') continue;
                int j = i;
                while (s[i] == 'D' && i < n) ++i;
                reverse(res.begin() + j, res.begin() + i + 1);
                --i;
            }
            return res;
        }
    };

    下面这种方法没有用到数组倒置,而是根据情况来往结果res中加入正确顺序的数字,我们遍历s字符串,遇到D直接跳过,遇到I进行处理,我们每次先记录下结果res的长度size,然后从i+1的位置开始往size遍历,将数字加入结果res中即可,参见代码如下:

    解法二:

    class Solution {
    public:
        vector<int> findPermutation(string s) {
            vector<int> res;
            for (int i = 0; i < s.size() + 1; ++i) {
                if (i == s.size() || s[i] == 'I') {
                    int size = res.size();
                    for (int j = i + 1; j > size; --j) {
                        res.push_back(j);
                    }
                }
            }
            return res;
        }
    };

    类似题目:

    Palindrome Permutation II

    Palindrome Permutation

    Permutation Sequence

    Permutations II

    Permutations

    Next Permutation

    参考资料:

    https://leetcode.com/problems/find-permutation/

    https://leetcode.com/problems/find-permutation/discuss/96644/c-simple-solution-in-72ms-and-9-lines

    https://leetcode.com/problems/find-permutation/discuss/96663/greedy-on-java-solution-with-explanation

    https://leetcode.com/problems/find-permutation/discuss/96613/java-on-clean-solution-easy-to-understand

    LeetCode All in One 题目讲解汇总(持续更新中...)

  • 相关阅读:
    机器学习问题
    sklearn学习笔记
    机器学习之广义线性模型
    因子学习笔记、问题汇总
    数学相关知识汇总
    下载技巧汇总
    python scipy库
    20180122 PyTorch学习资料汇总
    机器学习基本概念笔记
    机械学习框架、分类
  • 原文地址:https://www.cnblogs.com/grandyang/p/6366738.html
Copyright © 2011-2022 走看看