435. Non-overlapping Intervals (Medium)

Given a collection of intervals, find the minimum number of intervals you need to remove to make the rest of the intervals non-overlapping.

Note:

1. You may assume the interval's end point is always bigger than its start point.
2. Intervals like [1,2] and [2,3] have borders "touching" but they don't overlap each other.

Example 1:

Input: [ [1,2], [2,3], [3,4], [1,3] ]

Output: 1

Explanation: [1,3] can be removed and the rest of intervals are non-overlapping.

Example 2:

Input: [ [1,2], [1,2], [1,2] ]

Output: 2

Explanation: You need to remove two [1,2] to make the rest of intervals non-overlapping.

Example 3:

Input: [ [1,2], [2,3] ]

Output: 0

Explanation: You don't need to remove any of the intervals since they're already non-overlapping.

Solution: Greedy 9ms

这道题给了我们一堆区间，让我们求需要至少移除多少个区间才能使剩下的区间没有重叠，那么我们首先要给区间排序，根据每个区间的start来做升序排序，然后我们开始要查找重叠区间，判断方法是看如果前一个区间的end大于后一个区间的start，那么一定是重复区间，此时我们结果res自增1，我们需要删除一个，那么此时我们究竟该删哪一个呢，为了保证我们总体去掉的区间数最小，我们去掉那个end值较大的区间，而在代码中，我们并没有真正的删掉某一个区间，而是用一个变量last指向上一个需要比较的区间，我们将last指向end值较小的那个区间；如果两个区间没有重叠，那么此时last指向当前区间，继续进行下一次遍历，参见代码如下

/**
 * 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:
    int eraseOverlapIntervals(vector<Interval>& intervals) {
        int last = 0, res = 0;
        sort(intervals.begin(), intervals.end(), [](Interval& a, Interval& b){ return a.start < b.start; });
        for (int i = 1; i < intervals.size(); ++i) {
            if (intervals[last].end <= intervals[i].start) { last = i; continue; } // no overlapping
            ++res;
            if (intervals[last].end > intervals[i].end) last = i;
        }
        return res;
    }
};