Leetcode解题: Merge Intervals (56)

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Given a collection of intervals, merge all overlapping intervals.

For example,
Given [1,3],[2,6],[8,10],[15,18],
return [1,6],[8,10],[15,18].

解法1：O(NlogN) Time

先排序之后再合并比较简单。只要检查后面的start是否比前面的end小就可以了。

Java
主要是锻炼一下Java中用Collections.sort(list, comparator)的用法。

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/**
 * Definition for an interval.
 * public class Interval {
 *     int start;
 *     int end;
 *     Interval() { start = 0; end = 0; }
 *     Interval(int s, int e) { start = s; end = e; }
 * }
 */
public class Solution {
    public List<Interval> merge(List<Interval> intervals) {
        if (intervals == null || intervals.size() == 0) {
            return intervals;
        }
        
        Comparator<Interval> comparator = new Comparator<Interval>() {
            public int compare(Interval left, Interval right) {
                return left.start - right.start;
            }
        };
        // Sort the interval based on the start 
        Collections.sort(intervals, comparator);
        
        List<Interval> res = new ArrayList<Interval>();
        res.add(intervals.get(0));
        for (int i = 1; i < intervals.size(); ++i) {
            Interval current = intervals.get(i);
            Interval previous = res.get(res.size() - 1);
            if (current.start <= previous.end) {
                previous.end = Math.max(current.end, previous.end);
            } else {
                res.add(current);
            }
        }
        return res;
    }
}