3. Longest Substring Without Repeating Characters LeetCode

 Time Complexity: O(N) 

class Solution {
	public:
	int lengthOfLongestSubstring(string s) {
	vector< int >cnt(256,0);
	int mx = 0;
	int st = 0;
	for(int i=0; i<(int)s.size(); i++){
	cnt[s[i]]++;
	if(cnt[ s[i] ] > 1){
	for(int j=st; j<i; j++){
	cnt[ s[j] ]--;
	if( s[i] == s[j] ){
	st = j+1;
	break;
	}
	}
	}
	mx = max(mx, i - st + 1);
	}
	return mx;
	}
	};

2. Add Two Numbers LeetCode

Time complexity: O(N)

/**
	* Definition for singly-linked list.
	* struct ListNode {
	* int val;
	* ListNode *next;
	* ListNode() : val(0), next(nullptr) {}
	* ListNode(int x) : val(x), next(nullptr) {}
	* ListNode(int x, ListNode *next) : val(x), next(next) {}
	* };
	*/
	class Solution {
	public:
	ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
	ListNode* root = new ListNode(0);
	ListNode* ret = root;
	int carry = 0;
	while(l1 or l2){
	int sum = carry;
	if(l1){
	sum+= l1->val;
	l1=l1->next;
	}
	if(l2){
	sum+= l2->val;
	l2=l2->next;
	}
	carry = sum / 10;
	sum = sum % 10;
	ret->next = new ListNode(sum);
	ret = ret->next;
	}
	while(l1){
	int sum = carry + l1->val;
	l1=l1->next;
	carry = sum / 10;
	sum = sum % 10;
	ret->next = new ListNode(sum);
	ret = ret->next;
	}
	while(l2){
	int sum = carry + l2->val;
	l2=l2->next;
	carry = sum / 10;
	sum = sum % 10;
	ret->next = new ListNode(sum);
	ret = ret->next;
	}
	if(carry){
	ret->next = new ListNode(carry);
	ret = ret->next;
	}
	return root->next;
	}
	};

1. Two Sum LeetCode

C++ STL map solution O(nlogn). 

class Solution {
	public:
	vector<int> twoSum(vector<int>& nums, int target) {
	map< int ,int > mp;
	vector< int > ans(2, -1);
	for(int i=0; i< (int)nums.size(); i++ ){
	if( mp.find( target - nums[i] ) != mp.end() ){
	return { i, mp[ target - nums[i] ] };
	}
	mp[ nums[i] ] = i;
	}
	return ans;
	}
	};