Two Pointers
Two pointer approach: When to use it?
1. When a given array is sorted
As long as a given array is sorted, two pointer approach is applicable.
In Two Integer Sum II, input array is sorted.
Sometimes let’s look into 3sum problem.
What I missed
It is always good to
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class Solution {
public:
// I should aim for a solution with O(n^2) time complexity and O(1) space
vector<vector<int>> threeSum(vector<int>& nums) {
vector<vector<int>> res;
// Sorting is allowed, because its time complexity is O(nlogn)
// And O(nlogn) < O(n^2)
sort(nums.begin(), nums.end());
int N = nums.size();
for (int i = 0; i < N-2; i++) {
// Remove duplicate triplets
if (i > 0 && nums[i] == nums[i-1]) {
continue;
}
int target = -1 * nums[i];
int left = i+1, right = N-1;
while (left < right) {
if (nums[left] + nums[right] == target) {
res.push_back({nums[i], nums[left], nums[right]});
left++;
while (left < right && nums[left] == nums[left-1]) {
left++;
}
}
else if (nums[left] + nums[right] < target) {
left++;
}
else {
right--;
}
}
}
return res;
}
};