static String findCommonString(String string1, String string2) {
		
		int length1 = string1.length();
		int length2 = string2.length();
		int[][] traceMap = new int[length1][length2];
		int maxNum = 0;
		int lastSubBegin = 0;
		String commonSub = "";
		
		if(length1==0 || length2==0) return "";
		for(int i=0;i<length1;i++) {
			for(int j=0;j<length2;j++) {
				if(string1.charAt(i)==string2.charAt(j)) {
					if(i==0 || j==0) {
						traceMap[i][j] = 1;
					} else {
						traceMap[i][j] = traceMap[i-1][j-1] + 1;
					}
					
					if(maxNum<traceMap[i][j]) {
						maxNum++;
						int thisSubBegin = i+1-traceMap[i][j];
						if(lastSubBegin==thisSubBegin) {
							commonSub += string1.charAt(i);
						} else {
							lastSubBegin = thisSubBegin;
							commonSub = string1.substring(thisSubBegin, i+1);
						}
					}
				}
			}
		}
		return commonSub;
		
	}