56. Merge Intervals

问题描述：

Given a collection of intervals, merge all overlapping intervals.

Example 1:

Input: [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: Since intervals [1,3] and [2,6] overlaps, merge them into [1,6].

Example 2:

Input: [[1,4],[4,5]]
Output: [[1,5]]
Explanation: Intervals [1,4] and [4,5] are considerred overlapping.

解题思路：

现将给出的数组按照start的大小由小到大排序。

将第一个压入返回数组。

比较数组中最后一个Interval的end与当前interval的start的关系

代码：

/**
 * Definition for an interval.
 * struct Interval {
 *     int start;
 *     int end;
 *     Interval() : start(0), end(0) {}
 *     Interval(int s, int e) : start(s), end(e) {}
 * };
 */
struct cmp{
    bool operator() (const Interval &i1, const Interval &i2){
        return i1.start < i2.start;
    }
};
class Solution {
public:
    vector<Interval> merge(vector<Interval>& intervals) {
        vector<Interval> ret;
        if(intervals.size() < 2) return intervals;
        sort(intervals.begin(), intervals.end(), cmp() );
        ret.push_back(intervals[0]);
        for(int i = 1; i < intervals.size(); i++){
            if(intervals[i].start > ret.back().end){
                ret.push_back(intervals[i]);
            }else{
                ret.back().end = max(ret.back().end, intervals[i].end);
            }
        }
        
        return ret;
    }
};