611. Valid Triangle Number**

https://leetcode.com/problems/valid-triangle-number/description/

题目描述

Given an array consists of non-negative integers, your task is to count the number of triplets chosen from the array that can make triangles if we take them as side lengths of a triangle.
Example 1:

Input: [2,2,3,4]
Output: 3
Explanation:
Valid combinations are: 
2,3,4 (using the first 2)
2,3,4 (using the second 2)
2,2,3

Note:

The length of the given array won’t exceed 1000.
The integers in the given array are in the range of [0, 1000].

解题思路

对数组进行从小到大排序, 指针 i 指向最长边, 然后在 [0, i - 1] 范围内搜索 l 和 r 使得 nums[l] + nums[r] > nums[i].

C++ 实现 1

class Solution {
public:
    int triangleNumber(vector<int>& nums) {
        if (nums.size() < 3) return 0;
        int count = 0;
        std::sort(nums.begin(), nums.end());
        for (int i = nums.size() - 1; i >= 2; --i) {
            int l = 0, r = i - 1;
            while (l < r) {
                if (nums[l] + nums[r] > nums[i]) {
                    count += r - l;
                    -- r;
                } else ++ l;
            }
        }
        return count;
    }
};

C++ 实现 2

class Solution {
public:
    int triangleNumber(vector<int>& nums) {
        if (nums.size() < 3) return 0;
        int count = 0;
        std::sort(nums.begin(), nums.end());
        // 三元组 (i, j, k), 那么 i 最多到 nums.size() - 3
      	// k 的初始值为 i + 2. k 最多移动到 nusm.size() - 1;
      	// 当确定 j 之后, 使用 while 循环知道第一个不满足 > 条件的 k,
      	// 那么满足条件的三角形就是 k - j - 1.
      	// 注意 k = i + 2 放置的位置, 当 j 发生变化时, nums[i+2,...,k-1]
      	// 肯定满足 nums[i] + nums[j] > nums[k] 的, 所以不需要重复累加.
        for (int i = 0; i < nums.size() - 2; ++i) {
            int k = i + 2;
            for (int j = i + 1; j < nums.size() && 
                        nums[i] != 0 &&
                        nums[j] != 0; ++j) {
                while (k < nums.size() && nums[i] + nums[j] > nums[k]) ++k;
                count += k - j - 1;
            }
        }
        return count;
    }
};

来源：CSDN

作者：珍妮的选择

链接：https://blog.csdn.net/Eric_1993/article/details/103588186

标签

triangle