翻转字符串/左旋转字符串/翻转单词顺序列

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例如：AAAbbbccc左旋为bbbcccAAA

//非常规做法，牛客网accept
class Solution {
public:
	string LeftRotateString(string str, int n) {
		if (n>str.size()||n<0)return "";
		if (n == str.size()||n==0)return str;
		string s1, s2;
		for (int i = 0; i < str.size();i++)
		if (i < n)s1 += str[i];
		else s2 += str[i];
		return s2 + s1;
	}
};

2.原理：输入 $AB$---------------------------------------输出 $BA$.
$(A^{T}B^{T})^{T}=BA$

//面试考点翻转字符串
class Solution {
public:
	//定义一个翻转函数
	string RevStr(string &str,int start,int end)
	{
		while (start<end)
		{
			swap(str[start], str[end]);
			start++; end--;
		}
			return str;
	}
	string LeftRotateString(string str, int n) {
		int len = str.length();
		if (n>len||n<0)return "";
		if (n == len||n==0)return str;
		RevStr(str, 0, n - 1);
		RevStr(str, n, len - 1);
		RevStr(str, 0, len - 1);
		return str;
	}
};

3.翻转单词顺序列

class Solution {
public:
	//时间复杂度O(n)
	void ReverseWord(string &str,int low ,int high)
	{
		while (low<high)
		{
			swap(str[low], str[high]);
			low++;
			high--;
		}
	}
	string ReverseSentence(string str) {
		int len = str.size();
		if (len<1)return "";
		ReverseWord(str,0,str.size()-1);
		int low = 0, high = 0;

		while (high< len)
		{
			while (high < len&&str[high] == ' ')high++;//跳过空格
			low = high;//重新起始点
			while (high<len&&str[high] != ' ')
				high++;
			ReverseWord(str, low, high - 1);
		}
		return str;			
	}
};