Question

436. Find Right Interval

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Description

You are given an array of intervals, where intervals[i] = [starti, endi] and each starti is unique.

The right interval for an interval i is an interval j such that startj >= endi and startj is minimized. Note that i may equal j.

Return _an array of right interval indices for each interval i_. If no right interval exists for interval i, then put -1 at index i.

 

Example 1:

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

Example 2:

Input: intervals = [[3,4],[2,3],[1,2]]
Output: [-1,0,1]
Explanation: There is no right interval for [3,4].
The right interval for [2,3] is [3,4] since start0 = 3 is the smallest start that is >= end1 = 3.
The right interval for [1,2] is [2,3] since start1 = 2 is the smallest start that is >= end2 = 2.

Example 3:

Input: intervals = [[1,4],[2,3],[3,4]]
Output: [-1,2,-1]
Explanation: There is no right interval for [1,4] and [3,4].
The right interval for [2,3] is [3,4] since start2 = 3 is the smallest start that is >= end1 = 3.

 

Constraints:

• 1 <= intervals.length <= 2 * 104
• intervals[i].length == 2
• -106 <= starti <= endi <= 106
• The start point of each interval is unique.

Solutions

Solution 1

class Solution:
  def findRightInterval(self, intervals: List[List[int]]) -> List[int]:
    for i, v in enumerate(intervals):
      v.append(i)
    intervals.sort()
    n = len(intervals)
    ans = [-1] * n
    for _, e, i in intervals:
      j = bisect_left(intervals, [e])
      if j < n:
        ans[i] = intervals[j][2]
    return ans

class Solution {
  public int[] findRightInterval(int[][] intervals) {
    int n = intervals.length;
    List<int[]> starts = new ArrayList<>();
    for (int i = 0; i < n; ++i) {
      starts.add(new int[] {intervals[i][0], i});
    }
    starts.sort(Comparator.comparingInt(a -> a[0]));
    int[] res = new int[n];
    int i = 0;
    for (int[] interval : intervals) {
      int left = 0, right = n - 1;
      int end = interval[1];
      while (left < right) {
        int mid = (left + right) >> 1;
        if (starts.get(mid)[0] >= end) {
          right = mid;
        } else {
          left = mid + 1;
        }
      }
      res[i++] = starts.get(left)[0] < end ? -1 : starts.get(left)[1];
    }
    return res;
  }
}

class Solution {
public:
  vector<int> findRightInterval(vector<vector<int>>& intervals) {
    int n = intervals.size();
    vector<pair<int, int>> starts;
    for (int i = 0; i < n; ++i) {
      starts.emplace_back(make_pair(intervals[i][0], i));
    }
    sort(starts.begin(), starts.end());
    vector<int> res;
    for (auto interval : intervals) {
      int left = 0, right = n - 1;
      int end = interval[1];
      while (left < right) {
        int mid = left + right >> 1;
        if (starts[mid].first >= end)
          right = mid;
        else
          left = mid + 1;
      }
      res.push_back(starts[left].first < end ? -1 : starts[left].second);
    }
    return res;
  }
};

func findRightInterval(intervals [][]int) []int {
  n := len(intervals)
  starts := make([][]int, n)
  for i := 0; i < n; i++ {
    starts[i] = make([]int, 2)
    starts[i][0] = intervals[i][0]
    starts[i][1] = i
  }
  sort.Slice(starts, func(i, j int) bool {
    return starts[i][0] < starts[j][0]
  })
  var res []int
  for _, interval := range intervals {
    left, right, end := 0, n-1, interval[1]
    for left < right {
      mid := (left + right) >> 1
      if starts[mid][0] >= end {
        right = mid
      } else {
        left = mid + 1
      }
    }
    val := -1
    if starts[left][0] >= end {
      val = starts[left][1]
    }
    res = append(res, val)
  }
  return res
}

function findRightInterval(intervals: number[][]): number[] {
  const n = intervals.length;
  const starts = Array.from({ length: n }).map(() => new Array<number>(2));
  for (let i = 0; i < n; i++) {
    starts[i][0] = intervals[i][0];
    starts[i][1] = i;
  }
  starts.sort((a, b) => a[0] - b[0]);

  return intervals.map(([_, target]) => {
    let left = 0;
    let right = n;
    while (left < right) {
      const mid = (left + right) >>> 1;
      if (starts[mid][0] < target) {
        left = mid + 1;
      } else {
        right = mid;
      }
    }
    if (left >= n) {
      return -1;
    }
    return starts[left][1];
  });
}