## 题目描述：

LeetCode 830. Positions of Large Groups

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]]```

Note:  `1 <= S.length <= 1000`

## Python代码：

``````class Solution(object):
def largeGroupPositions(self, S):
"""
:type S: str
:rtype: List[List[int]]
"""
j = -1
d = ''
ans = []
for i, c in enumerate(S + '#'):
if c != d:
if i - j >= 3:
ans.append([j, i - 1])
j = i
d = c
return ans
``````

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