题目：

In a string S of lowercase letters, these letters form consecutive groups of the same character.

For example, a string like S = “abbxxxxzyy” has the groups “a”, “bb”, “xxxx”, “z” and “yy”.

Call a group large if it has 3 or more characters. We would like the starting and ending positions of every large group.

The final answer should be in lexicographic order.

Example 1:

Input: "abbxxxxzzy"
Output: [[3,6]]
Explanation: "xxxx" is the single large group with starting  3 and ending positions 6.

Example 2:

Input: "abc"
Output: []
Explanation: We have "a","b" and "c" but no large group.

Example 3:

Input: "abcdddeeeeaabbbcd"
Output: [[3,5],[6,9],[12,14]]

分析:

要我们找出给定字符串中，所有的连续字符（长度>=3）的位置。

双指针移动，注意最后的边界判断

JavaCode:

class Solution {
    public List<List<Integer>> largeGroupPositions(String S) {
        ArrayList<List<Integer>> ret=new ArrayList<List<Integer>>();
        int st=0,ed=1;
        while(ed<S.length()){
            if(S.charAt(ed)==S.charAt(st)){
                ed++;
            }
            else{
                if(ed-st>=3){
                    ArrayList<Integer> tmp=new ArrayList<Integer>();
                    tmp.add(st);
                    tmp.add(ed-1);
                    ret.add(tmp);
                }
                st=ed;
                ed=ed+1;
            }
        }
        if(ed-st>=3){
            ArrayList<Integer> tmp=new ArrayList<Integer>();
            tmp.add(st);
            tmp.add(ed-1);
            ret.add(tmp);
        }
        return ret;
    }
}