611 Valid Triangle Number
1. Question
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
2. Implementation
(1) Two Pointer
class Solution {
public int triangleNumber(int[] nums) {
if (nums == null || nums.length <= 2) {
return 0;
}
Arrays.sort(nums);
int count = 0;
for (int i = 0; i < nums.length - 2; i++) {
int k = i + 2;
for (int j = i + 1; nums[i] != 0 && j < nums.length - 1; j++) {
while (k < nums.length && nums[i] + nums[j] > nums[k]) {
k++;
}
count += k - j - 1;
}
}
return count;
}
}
3. Time & Space Complexity
Two Pointer: 时间复杂度O(n^2), 空间复杂度O(1),因为sorting需要空间
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