vault backup: 2022-09-01 14:14:08

OJ notes/pages/Leetcode Merge-Intervals.md

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+# Leetcode Merge-Intervals
+
+#### 2022-09-01 14:04
+
+> ##### Algorithms:
+> #algorithm #sort 
+> ##### Data structures:
+> #DS #array 
+> ##### Difficulty:
+> #coding_problem #difficulty-medium 
+> ##### Additional tags:
+> #leetcode 
+> ##### Revisions:
+> N/A
+
+##### Links:
+- [Link to problem](https://leetcode.com/problems/merge-intervals/)
+- [Solution ans Explanation](https://leetcode.com/problems/merge-intervals/solution/)
+___
+### Problem
+
+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_.
+
+#### Examples
+
+**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.
+
+#### Constraints
+
+-   `1 <= intervals.length <= 104`
+-   `intervals[i].length == 2`
+-   `0 <= starti <= endi <= 104`
+
+### Thoughts
+
+> [!summary]
+> This is a generic array problem.
+
+#### Situations to consider:
+
+- The intervals can be unordered
+- The first interval
+- `[0, 3] [0, 1]` are adjacent and overlapped.
+
+To solve the situations, sort first, and use `max` function to solve the 3rd solution.
+
+### Solution
+
+```cpp
+class Solution {
+public:
+  vector<vector<int>> merge(vector<vector<int>> &intervals) {
+    // sort first, so that data are continious
+    vector<vector<int>> ans;
+
+    sort(intervals.begin(), intervals.end());
+
+    for (auto interval : intervals) {
+      if (ans.empty() || ans.back()[1] < interval[0]) {
+        ans.push_back(interval);
+      } else {
+        ans.back()[1] = max(interval[1], ans.back()[1]);
+      }
+    }
+
+    return ans;
+  }
+};
+```