Find Right Interval问题及解法

问题描述：

Given a set of intervals, for each of the interval i, check if there exists an interval j whose start point is bigger than or equal to the end point of the interval i, which can be called that j is on the "right" of i.

For any interval i, you need to store the minimum interval j's index, which means that the interval j has the minimum start point to build the "right" relationship for interval i. If the interval j doesn't exist, store -1 for the interval i. Finally, you need output the stored value of each interval as an array.

Note:

1. You may assume the interval's end point is always bigger than its start point.
2. You may assume none of these intervals have the same start point.

示例:

Input: [ [1,2] ]Output: [-1]Explanation: There is only one interval in the collection, so it outputs -1.

Input: [ [3,4], [2,3], [1,2] ]Output: [-1, 0, 1]Explanation: There is no satisfied "right" interval for [3,4].For [2,3], the interval [3,4] has minimum-"right" start point;For [1,2], the interval [2,3] has minimum-"right" start point.

Input: [ [1,4], [2,3], [3,4] ]Output: [-1, 2, -1]Explanation: There is no satisfied "right" interval for [1,4] and [3,4].For [2,3], the interval [3,4] has minimum-"right" start point.

问题分析：

本题主要是寻找interval的start，根据题意，我们可以将各个start升序排序，利用lower_bound()二分查找满足条件的start。

过程详见代码：

/** * Definition for an interval. * struct Interval { *     int start; *     int end; *     Interval() : start(0), end(0) {} *     Interval(int s, int e) : start(s), end(e) {} * }; */#define ii pair<int,int>struct cmp{bool operator()(const ii& first, const ii& second) const{return first.first < second.first;}};class Solution {public:    vector<int> findRightInterval(vector<Interval>& intervals) {        vector<ii> vt;int i = 0;for (auto it : intervals){vt.push_back(ii(it.start, i));i++;}sort(vt.begin(), vt.end());vector<int> ret;for (auto it : intervals){auto it1 = lower_bound(vt.begin(),vt.end(), ii(it.end, -1));int res = (it1 != vt.end()) ? it1->second : -1;ret.push_back(res);}return ret;    }};