LeetCode: Interval

(1)Merge Intervals

https://leetcode.com/problems/merge-intervals/

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].

思路：贪心思想。首先根据intervals的start进行排序。排完序之后，先将第一个interval加入vector，接着判断之前加入vector的interval和后面一个interval比较。因为是排好序的，所以，只需要判断新的interval的start是否大于前面一个加入vector的interval的end。如果大于，则将后面的interval加入vector，如果不大于，则肯定有交集，两者合并。

struct Interval{
    int start;
    int end;
    Interval():start(0),end(0){}
    Interval(int s,int e):start(s),end(e){}
};
bool cmp(const Interval &left,const Interval & right){
        if (left.start == right.start){
                return left.end < right.end;
        }else{
                return left.start < right.start;
        }
}
class Solution {
public:
    /*struct cmp{
        bool operator()(const Interval &left,const Interval & right){
            if (left.start == right.start){
                return left.end < right.end;
            }else{
                return left.start < right.start;
            }
        }
    };*/
    
    vector<Interval> merge(vector<Interval> &intervals) {
        vector<Interval> result;
        if (intervals.size() == 0 || intervals.empty()){
            return result;
        }
        sort(intervals.begin(),intervals.end(),cmp);
        result.push_back(intervals[0]);
        int index = 0;
        for (int i = 1; i < intervals.size(); i++){
            if (intervals[i].start <= result[index].end){
                int end = max(intervals[i].end,result[index].end);
                result[index].end = end;
                //index++;
                //result.push_back(intervals[i]);
            }else{
                result.push_back(intervals[i]);
                index++;
            }
        }
        return result;
    }
};

（2）Insert Interval

https://leetcode.com/problems/insert-interval/

Title：

Given a set of non-overlapping intervals, insert a new interval into the intervals (merge if necessary).

You may assume that the intervals were initially sorted according to their start times.

Example 1:
Given intervals [1,3],[6,9], insert and merge [2,5] in as [1,5],[6,9].

Example 2:
Given [1,2],[3,5],[6,7],[8,10],[12,16], insert and merge [4,9] in as [1,2],[3,10],[12,16].

This is because the new interval [4,9] overlaps with [3,5],[6,7],[8,10].

思路：我最直接的想法就是先找到newInterval的start在哪两个intervals之间，然后再找到end。虽然也能做，但是代码很复杂，不精练

class Solution {
public:
    vector<Interval> insert(vector<Interval> & intervals,
            Interval newInterval) {
        vector<Interval> result;
        if (intervals.size() == 0 || intervals.empty()){
            result.push_back(newInterval);
            return result;
        }
        int i;
        bool flag = false;
        for (i = 0; i < intervals.size();) {
            if (intervals[i].start >= newInterval.start) {
                int start, end;
                if (i == 0) {
                    start = newInterval.start;
                } else {
                    if (intervals[i-1].end >= newInterval.start){
                        start = intervals[i - 1].start;
                        result.pop_back();
                    }else
                        start = newInterval.start;
                    
                    
                }
                while (i < intervals.size() && intervals[i].start <= newInterval.end) {
                    i++;
                }
                if(i == intervals.size()){
                    end = max(intervals[i-1].end,newInterval.end);
                    Interval interval(start,end);
                    result.push_back(interval);
                }else{
                    end = max(intervals[i - 1].end, newInterval.end);
                    Interval interval(start, end);
                    result.push_back(interval);
                    for (int j = i; j < intervals.size(); j++)
                        result.push_back(intervals[j]);
                }
                flag = true;
                break;
            } else {
                result.push_back(intervals[i]);
                i++;
            }
        }
        if (!flag){
            if (intervals[i-1].end < newInterval.start){
                //result.push_back(intervals[i-1
                result.push_back(newInterval);
            }else{
                result.pop_back();
                int start = intervals[i-1].start;
                int end = max(intervals[i-1].end,newInterval.end);
                Interval interval(start,end);
                result.push_back(interval);
            }
        }
        
        return result;
    }

};

然后看了网上其他人的想法

Quickly summarize 3 cases. Whenever there is intersection, created a new interval.

遍历一遍vector，根据上述三种情况考虑添加。

vector<Interval> insert(vector<Interval> &intervals, Interval newInterval){
        vector<Interval> result;
        for (int i = 0 ; i < intervals.size(); i++){
            if (intervals[i].end < newInterval.start){
                result.push_back(intervals[i]);
            }else if (intervals[i].start > newInterval.end){
                result.push_back(newInterval);
                newInterval = intervals[i];
            }else{
                newInterval = Interval(min(intervals[i].start,newInterval.start),max(intervals[i].end,newInterval.end));
            }
        }
        result.push_back(newInterval);
        return result;
    }