Press "Enter" to skip to content

Posts tagged as “Medium”

LeetCode 15. 3Sum (javascript)

By stone on March 24, 2021

Given an array nums of n integers, are there elements a, b, c in nums such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Notice that the solution set must not contain duplicate triplets.

Example 1:

Input: nums = [-1,0,1,2,-1,-4]
Output: [[-1,-1,2],[-1,0,1]]

Example 2:

Input: nums = []
Output: []

Example 3:

Input: nums = [0]
Output: []

Constraints:

  • 0 <= nums.length <= 3000
  • -105 <= nums[i] <= 105

Idea:

  • Sort nums + two pointers
  • Enumerate nums[i]
  • Use two pointers to find all possible sets of (i,  l,  r) such that
    • i < l < r
    • nums[i] + nums[l] + nums[r] === 0
  • How to move pointers?
    • nums[i] + nums[l] + nums[r] > 0, too large, decrease r
    • nums[i] + nums[l] + nums[r] > 0, too small, increase l

Optimize:

  • skip: if nums[i] > 0, then nums[i] + nums[l] + nums[r] never = 0 
  • skip same number: nums[i] === nums[i – 1]

Solution:

Time complexity: O(nlogn + n^2)

Space complexity: O(1)

/**
 * @param {number[]} nums
 * @return {number[][]}
 */
var threeSum = function(nums) {
    const n = nums.length;
    let res = [];
    nums.sort((a, b) => a - b);
    for (let i = 0; i < n - 2; i++) {
        // skip: if nums[i] > 0,
        // then nums[i] + nums[l] + nums[r] never = 0
        if (nums[i] > 0) break; 
        // skip same number
        if (i > 0 && nums[i] === nums[i - 1]) continue;
        let l = i + 1;
        let r = n - 1;
        while (l < r) {
            if (nums[i] + nums[l] + nums[r] === 0) {
                res.push([nums[i], nums[l++], nums[r--]]);
                // skip same number
                while (l < r && nums[l] === nums[l - 1]) l++;
                while (l < r && nums[r] === nums[r + 1]) r--;
            } else if (nums[i] + nums[l] + nums[r] > 0) {
                r--;
            } else {          
                l++;
            }  
        }
    }
    return res;
};

LeetCode 11. Container With Most Water (javascript)

By stone on March 19, 2021

Given n non-negative integers a1, a2, ..., an, where each represents a point at coordinate (i, ai). n vertical lines are drawn such that the two endpoints of the line i is at (i, ai) and (i, 0). Find two lines, which, together with the x-axis forms a container, such that the container contains the most water.

Notice that you may not slant the container.

Example 1:

Input: height = [1,8,6,2,5,4,8,3,7]
Output: 49
Explanation: The above vertical lines are represented by array [1,8,6,2,5,4,8,3,7]. In this case, the max area of water (blue section) the container can contain is 49.

Example 2:

Input: height = [1,1]
Output: 1

Example 3:

Input: height = [4,3,2,1,4]
Output: 16

Example 4:

Input: height = [1,2,1]
Output: 2

Constraints:

  • n == height.length
  • 2 <= n <= 105
  • 0 <= height[i] <= 104

Solution:

/**
 * @param {number[]} height
 * @return {number}
 */
var maxArea = function(height) {
    let len = height.length;
    let l = 0
    let r = len - 1;
    let maxA = 0;
    while (l < r) {
        let minH = Math.min(height[l], height[r]);
        maxA = Math.max(maxA, minH * (r - l));
        if (height[l] < height[r]) {
            l++;
        } else {
            r--;
        }
    }
    return maxA;
};

LeetCode 3. Longest Substring Without Repeating Characters (javascript)

By stone on March 19, 2021

Given a string s, find the length of the longest substring without repeating characters.

Example 1:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

Example 2:

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

Example 4:

Input: s = ""
Output: 0

Constraints:

  • 0 <= s.length <= 5 * 104
  • s consists of English letters, digits, symbols and spaces.

Solutions:

/**
 * @param {string} s
 * @return {number}
 */
var lengthOfLongestSubstring = function(s) {
    let n = s.length;
    let ans = 0;
    let visited = new Map();
    for (let i = 0; i < n; i++) {
        for (let j = i; j < n ; j++) {
            if (visited.has(s[j])) {
                visited.clear();
                break;
            } else {
                ans = Math.max(ans, j - i + 1);
                visited.set(s[j], true);
            }
            
        }
        
    }
    return ans;
};

LeetCode 19. Remove Nth Node From End of List (Javascript)

By stone on March 19, 2021

Given the head of a linked list, remove the nth node from the end of the list and return its head.

Follow up: Could you do this in one pass?

Example 1:

Input: head = [1,2,3,4,5], n = 2
Output: [1,2,3,5]

Example 2:

Input: head = [1], n = 1
Output: []

Example 3:

Input: head = [1,2], n = 1
Output: [1]

Constraints:

  • The number of nodes in the list is sz.
  • 1 <= sz <= 30
  • 0 <= Node.val <= 100
  • 1 <= n <= sz
/**
 * Definition for singly-linked list.
 * function ListNode(val, next) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.next = (next===undefined ? null : next)
 * }
 */
/**
 * @param {ListNode} head
 * @param {number} n
 * @return {ListNode}
 */
var removeNthFromEnd = function(head, n) {
    if (head.next === null) 
        return null;
    
    let first = new ListNode();
    first.next = head;
    let second = first;
    // first point to n + 1
    first = first.next;
    while (n > 0) {
        first = first.next;
        n--;
    }
    // first already reach to the end null, means need to remove the first node
    if (first === null) {
        head = head.next;
    } else {
        // maintaining N nodes apart between first and second pointer
        while (first !== null) {
            first = first.next;
            second = second.next;
        }
        // Now second pointer to the node that need to be removed
        // remove that node now
        second.next = second.next.next;
    }

    return head;
    
};

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;
};