leetcode 611. Valid Triangle Number 构成三角形数量 + 双指针

灰太狼 2022-06-03 05:13 104阅读 0赞

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 \[0, 1000\].

排序之后，从数字末尾开始往前遍历，将left指向首数字，将right之前遍历到的数字的前面一个数字，然后如果left小于right就进行循环，循环里面判断如果left指向的数加上right指向的数大于当前的数字的话，那么right到left之间的数字都可以组成三角形，这是为啥呢，相当于此时确定了i和right的位置，可以将left向右移到right的位置，中间经过的数都大于left指向的数，所以都能组成三角形，就说这思路叼不叼！加完之后，right自减一，即向左移动一位。如果left和right指向的数字之和不大于nums\[i\]，那么left自增1，即向右移动一位，参见代码如下：

代码如下：

#include <iostream>
    #include <vector>
    #include <map>
    #include <unordered_map>
    #include <set>
    #include <unordered_set>
    #include <queue>
    #include <stack>
    #include <string>
    #include <climits>
    #include <algorithm>
    #include <sstream>
    #include <functional>
    #include <bitset>
    #include <numeric>
    #include <cmath>
    #include <regex>
    #include <iomanip>
    #include <cstdlib>
    #include <ctime>
    
    using namespace std;
    
    #include <iostream>
    #include <vector>
    #include <map>
    #include <unordered_map>
    #include <set>
    #include <unordered_set>
    #include <queue>
    #include <stack>
    #include <string>
    #include <climits>
    #include <algorithm>
    #include <sstream>
    #include <functional>
    #include <bitset>
    #include <numeric>
    #include <cmath>
    #include <regex>
    #include <iomanip>
    #include <cstdlib>
    #include <ctime>
    
    using namespace std;
    
    
    class Solution 
    {
    public:
        int triangleNumber(vector<int>& n) 
        {
            int res = 0;
            sort(n.begin(), n.end());
            for (int i = n.size() - 1; i >= 2; i--)
            {
                int left = 0, right = i - 1;
                while (left <= right)
                {
                    if (n[left] + n[right] > n[i])
                    {
                        res += (right - 1 - left + 1);
                        right--;
                    }
                    else
                        left++;
                }
            }
            return res;
        }
    };