56. Merge Intervals

• Difficulty: Medium

• Topics: Array, Sort

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Problem:

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 considered overlapping.

NOTE: input types have been changed on April 15, 2019. Please reset to default code definition to get new method signature.

Solutions:

class Solution {
public:
    vector<vector<int>> merge(vector<vector<int>>& intervals) {
        vector<vector<int>> ret;
        sort(intervals.begin(), intervals.end()); // be careful about vector comparison
        for (auto& interval : intervals) {
            if (ret.empty() || ret.back()[1] < interval[0]) {
                ret.push_back(interval);
            } else {
                ret.back()[1] = max(ret.back()[1], interval[1]);
            }
        }

        return ret;
    }
};