85 lines
1.4 KiB
Markdown
85 lines
1.4 KiB
Markdown
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# Leetcode Largest-Perimeter-Triangle
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#### 2022-07-26 08:39
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> ##### Algorithms:
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>
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> #algorithm #greedy #math
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>
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> ##### Difficulty:
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>
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> #coding_problems #difficulty_easy
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>
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> ##### Additional tags:
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>
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> #leetcode
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>
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> ##### Revisions:
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>
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> N/A
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##### Related topics:
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##### Links:
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- [Link to problem](https://leetcode.com/problems/largest-perimeter-triangle/)
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***
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### Problem
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Given an integer array `nums`, return _the largest perimeter of a triangle with a non-zero area, formed from three of these lengths_. If it is impossible to form any triangle of a non-zero area, return `0`.
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#### Examples
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**Example 1:**
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**Input:** nums = [2,1,2]
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**Output:** 5
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**Example 2:**
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**Input:** nums = [1,2,1]
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**Output:** 0
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#### Constraints
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- `3 <= nums.length <= 104`
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- `1 <= nums[i] <= 106`
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### Thoughts
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> [!summary]
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> This is a #math problem.
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After sorting, the answer must be consecutive 3 elements.
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#### Why?
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assume l, m , r.
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if nums[l] + nums[m] < nums[r], because after sorting,
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the order is ascending, anything before l and m will not suffice.
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### Solution
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```cpp
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class Solution {
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public:
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int largestPerimeter(vector<int> &nums) {
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sort(nums.begin(), nums.end());
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int r = nums.size() - 1;
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// change l first until its boundary
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for (int i = r; i > 1; i--) {
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if (nums[i] < nums[i - 1] + nums[i - 2]) {
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return nums[i] + nums[i - 1] + nums[i - 2];
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}
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}
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return 0;
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}
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};
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```
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