LeetCode #56 — MEDIUM

Merge Intervals

Move from brute-force thinking to an efficient approach using array strategy.

The Problem

Problem Statement

Given an array of intervals where intervals[i] = [start_i, end_i], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.

Example 1:

Input: intervals = [[1,3],[2,6],[8,10],[15,18]]
Output: [[1,6],[8,10],[15,18]]
Explanation: Since intervals [1,3] and [2,6] overlap, merge them into [1,6].

Example 2:

Input: intervals = [[1,4],[4,5]]
Output: [[1,5]]
Explanation: Intervals [1,4] and [4,5] are considered overlapping.

Example 3:

Input: intervals = [[4,7],[1,4]]
Output: [[1,7]]
Explanation: Intervals [1,4] and [4,7] are considered overlapping.

Constraints:

1 <= intervals.length <= 10⁴
intervals[i].length == 2
0 <= start_i <= end_i <= 10⁴

Step 01

Brute Force Baseline

Problem summary: Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input.

Baseline thinking

Start with the most direct exhaustive search. That gives a correctness anchor before optimizing.

Pattern signal: Array

Example 1

[[1,3],[2,6],[8,10],[15,18]]

Example 2

[[1,4],[4,5]]

Example 3

[[4,7],[1,4]]

Core Insight

What unlocks the optimal approach

No official hints in dataset. Start from constraints and look for a monotonic or reusable state.

Interview move: turn each hint into an invariant you can check after every iteration/recursion step.

Step 03

Algorithm Walkthrough

Iteration Checklist

Define state (indices, window, stack, map, DP cell, or recursion frame).
Apply one transition step and update the invariant.
Record answer candidate when condition is met.
Continue until all input is consumed.

Use the first example testcase as your mental trace to verify each transition.

Step 04

Edge Cases

Minimum Input

Single element / shortest valid input

Validate boundary behavior before entering the main loop or recursion.

Duplicates & Repeats

Repeated values / repeated states

Decide whether duplicates should be merged, skipped, or counted explicitly.

Extreme Constraints

Upper-end input sizes

Re-check complexity target against constraints to avoid time-limit issues.

Invalid / Corner Shape

Empty collections, zeros, or disconnected structures

Handle special-case structure before the core algorithm path.

Step 05

Full Annotated Code

import java.util.*;

class Solution {
    public int[][] merge(int[][] intervals) {
        Arrays.sort(intervals, (a, b) -> Integer.compare(a[0], b[0]));
        List<int[]> merged = new ArrayList<>();

        for (int[] cur : intervals) {
            if (merged.isEmpty() || merged.get(merged.size() - 1)[1] < cur[0]) {
                // No overlap: start a new interval.
                merged.add(new int[] { cur[0], cur[1] });
            } else {
                // Overlap: extend the end of the current merged interval.
                merged.get(merged.size() - 1)[1] =
                    Math.max(merged.get(merged.size() - 1)[1], cur[1]);
            }
        }

        return merged.toArray(new int[merged.size()][]);
    }
}

import "sort"

func merge(intervals [][]int) [][]int {
    sort.Slice(intervals, func(i, j int) bool {
        return intervals[i][0] < intervals[j][0]
    })

    merged := [][]int{}
    for _, cur := range intervals {
        if len(merged) == 0 || merged[len(merged)-1][1] < cur[0] {
            merged = append(merged, []int{cur[0], cur[1]})
        } else {
            if cur[1] > merged[len(merged)-1][1] {
                merged[len(merged)-1][1] = cur[1]
            }
        }
    }
    return merged
}

class Solution:
    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
        intervals.sort(key=lambda x: x[0])
        merged: List[List[int]] = []

        for start, end in intervals:
            if not merged or merged[-1][1] < start:
                merged.append([start, end])
            else:
                merged[-1][1] = max(merged[-1][1], end)

        return merged

impl Solution {
    pub fn merge(intervals: Vec<Vec<i32>>) -> Vec<Vec<i32>> {
        if intervals.is_empty() {
            return vec![];
        }

        let mut intervals = intervals;
        intervals.sort_by_key(|v| v[0]);
        let mut merged: Vec<Vec<i32>> = vec![intervals[0].clone()];

        for cur in intervals.into_iter().skip(1) {
            let last = merged.last_mut().unwrap();
            if cur[0] <= last[1] {
                last[1] = last[1].max(cur[1]);
            } else {
                merged.push(cur);
            }
        }

        merged
    }
}

function merge(intervals: number[][]): number[][] {
  intervals.sort((a, b) => a[0] - b[0]);
  const merged: number[][] = [];

  for (const cur of intervals) {
    if (merged.length === 0 || merged[merged.length - 1][1] < cur[0]) {
      merged.push([cur[0], cur[1]]);
    } else {
      merged[merged.length - 1][1] = Math.max(merged[merged.length - 1][1], cur[1]);
    }
  }

  return merged;
}

Step 06

Interactive Study Demo

Use this to step through a reusable interview workflow for this problem.

Custom checkpoints (one per line)

Press Step or Run All to begin.

Step 07

Complexity Analysis

Time

O(n × log n)

Space

O(log n)

Approach Breakdown

BRUTE FORCE

O(n²) time

O(1) space

Two nested loops check every pair or subarray. The outer loop fixes a starting point, the inner loop extends or searches. For n elements this gives up to n²/2 operations. No extra space, but the quadratic time is prohibitive for large inputs.

OPTIMIZED

O(n) time

O(1) space

Most array problems have an O(n²) brute force (nested loops) and an O(n) optimal (single pass with clever state tracking). The key is identifying what information to maintain as you scan: a running max, a prefix sum, a hash map of seen values, or two pointers.

Shortcut: If you are using nested loops on an array, there is almost always an O(n) solution. Look for the right auxiliary state.

Coach Notes

Common Mistakes

Review these before coding to avoid predictable interview regressions.

Off-by-one on range boundaries

Wrong move: Loop endpoints miss first/last candidate.

Usually fails on: Fails on minimal arrays and exact-boundary answers.

Fix: Re-derive loops from inclusive/exclusive ranges before coding.