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Posts published in “Difficulty”

LeetCode 2. Add Two Numbers

By stone on March 18, 2021

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4]

Output: [7,0,8]

Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0]

Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9]

Output: [8,9,9,9,0,0,0,1]

Constraints:

  • The number of nodes in each linked list is in the range [1, 100].
  • 0 <= Node.val <= 9
  • It is guaranteed that the list represents a number that does not have leading zeros.
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var addTwoNumbers = function(l1, l2) {
    let head = new ListNode();
    let temp = head;
    let sum = 0;
    let carry = 0;
    while (l1 !== null || l2 !== null) {
        sum = carry + (l1 !== null ? l1.val : 0) + (l2 !== null ? l2.val : 0);
        carry = sum >= 10 ? 1 : 0;
        sum = sum % 10;
        
        temp.next = new ListNode(sum);
        temp = temp.next;
        
        if (l1) l1 = l1.next;
        if (l2) l2 = l2.next;
        
        if (carry) temp.next = new ListNode(carry);
    }
    return head.next;
};

LeetCode 387. First Unique Character in a String

By stone on March 18, 2021

Given a string, find the first non-repeating character in it and return its index. If it doesn’t exist, return -1.

Examples:

s = "leetcode"
return 0.

s = "loveleetcode"
return 2.

Note: You may assume the string contains only lowercase English letters.

Idea: Hashtable

Solution:

/**
 * @param {string} s
 * @return {number}
 */
var firstUniqChar = function(s) {
    let map = new Map;
    
    for (let i = 0; i < s.length; i++) {
        if (map.has(s[i])) {
            map.set(s[i], map.get(s[i]) + 1);
        } else {
            map.set(s[i], 1);
        }
    }
    
    for (let i = 0; i < s.length; i++) {
        if (map.get(s[i]) === 1) return i;
    }
    return -1;
};

LeetCode 1. Two Sum

By stone on March 18, 2021

Problem:

Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Input: nums = [2,7,11,15], target = 9

Output: [0,1]

Output: Because nums[0] + nums[1] == 9, we return [0, 1].

Idea:

Hashtable

Solution:

Hashtable / Javascript

Time complexity: O(n)

Space complexity: O(n)

/**
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function(nums, target) {
    let map = new Map();
    
    for (let i = 0; i < nums.length; i++) {
        let diff = target - nums[i];
        if (map.has(diff)) {
            return [i, map.get(diff)];
        } else {
            map.set(nums[i],i);
        }
    }
};