56 Merge Intervals

题意：合并一系列有重合的区间。例如：Given [1,3],[2,6],[8,10],[15,18],    return [1,6],[8,10],[15,18].

分析：按start升序排列，再根据end合并。复杂度为O(nlogn).

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) {}
 * };
 */
class Solution {
public:
    vector<Interval> merge(vector<Interval>& intervals) {
        int n=intervals.size();
        if(n==0)  return intervals;
        sort(intervals.begin(),intervals.end(),cmp);
        vector<Interval> ans;
        Interval  v(intervals[0].start,intervals[0].end);
        for(int i=1;i<n;++i){
            if(intervals[i].start>v.end){
                ans.push_back(v);
                v.start=intervals[i].start;
                v.end=intervals[i].end;
            }else{
                v.end=max(v.end,intervals[i].end);
            }
        }
        ans.push_back(v);
        return ans;
    }
    static bool cmp(const Interval &a,const Interval &b){
        if(a.start==b.start)  return  a.end<b.end;
        else  return  a.start<b.start;
    }
};