找出字串的最長不重複子串，輸出長度和子串

方法一：窮舉法，空間複雜度是O(1)，時間複雜度是O(N^4)

java程式碼實現

<pre name="code" class="java">public class Max_substring {

	public int max_unique_substring(char[] str){
	int maxlen = 0;
	int begin = 0;
	int n = str.length;
	for(int i = 0; i < n; ++i){
		for(int j = i+1; j < n; ++j){
			int flag = 0;
			//判斷子串是否有重複字元
			for(int m = i; m < j; ++m){
				for(int k = m+1; k < j; ++k){
					if(str[m] == str[k]){
						flag = 1;
						break;
					}
				}
				if(1 == flag) break;				
			}
			if(0 == flag && j-i+1 > maxlen){
				maxlen = j-i+1;
				begin = i;
			}
		}
		
	}
	StringBuffer s = new StringBuffer();
	for(int g = 0; g < n; ++g){
		s.append(str[g]);
	}
	System.out.println(s.toString().substring(begin,begin+maxlen)+","+maxlen+","+ begin +","+(begin+maxlen));
	return maxlen;
	}
	
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Max_substring max = new Max_substring();
		char[] s = {'a','x','b','d','e','b','p','q','a','w','u','v','a'};
		max.max_unique_substring(s);
	}

}
 

方法二：貪心演算法，時間複雜度O(N)

public class Max_substring2 {

	public int max_substring(char[] str){
		
		int length = str.length;
		int begin = 0;    //記錄最長子串的起點  以便輸出最長串
		int end = 0;	//記錄最長子串的終點  以便輸出最長串
		int maxlen = 0;        //最長不重複字串長度
		int j = 0;			//記錄當前不重複字串的起點
		int i = 0;          //記錄當前不重複字串的終點
		int hs[] = new int[128];   //用陣列代替雜湊表map
		int k =0;
		
		/* 將模擬hashmap的陣列初始化*/
		while(k < 128){
			hs[k] = -1;
			k++;
		}
			
		//遍歷字元陣列
		while(i < length){     
			if(hs[str[i]-0] >= j){   //如果當前維護的不重複子串中出現了str[i]
				j = hs[str[i]-0]+1;  //更新j
			}else{ 					//如果當前維護的不重複子串沒有出現str[i]
				if(i-j+1 > maxlen){
					maxlen = i-j+1;   /*更新結果取較大者*/
					begin = j;		  /*  並記錄當前較大子串的起點*/
					end = i;		 /*  並記錄當前較大子串的終點*/
				}
			}
			hs[str[i]-0] = i;        //更新map(實際上是更新模擬map的陣列)
			i++;
			
		}
		//列印輸出最長串
		for(int m = begin; m <= end; ++m){
			System.out.print(str[m]);
		}
		
		return maxlen;
		
	}
	
	
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Max_substring2 mxs = new Max_substring2();
		char[] str = {'a','x','b','d','e','b','p','q','a','w','u','v','a'};
		int maxlen = mxs.max_substring(str);
		System.out.println(" 最大不重複子串的長度：  "+maxlen);
	}

}