[LeetCode] 830. Positions of Large Groups

轉自LeetCode

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

<Solution>

想法如下

• 通常 String 找區間的題目，可以先用雙指標來思考一下

• 先固定一個位置，然後開始歷遍。當遇到不同字元時，檢查當下的區間是不是符合 group 的定義，是的話就記錄下來。然後不論區間是不是答案之一，都要更新固定的位置，以用來做新的比對

code 如下

Java(參考解法)

	class Solution {
	public List<List<Integer>> largeGroupPositions(String S) {
	List<List<Integer>> ans = new ArrayList<>();
	final int len = S.length();
	int anchor = 0, upperBound = len - 1;
	for(int i = 0; i < len; i++) {
	if(i == upperBound || S.charAt(i) != S.charAt(i+1)) {
	if(i - anchor >= 2) {
	ans.add(Arrays.asList(new Integer[]{anchor, i}));
	}
	anchor = i+1;
	}
	}
	return ans;
	}
	}

view raw largeGroupPositions.java hosted with ❤ by GitHub