023. Merge k Sorted Lists

Difficulty: Hard

Frequency: N/A

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

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/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
private:
    struct compare {
        bool operator() (const ListNode* l, const ListNode* r) {
            return l->val > r->val;
        }
    };
public:
    ListNode* mergeKLists(vector<ListNode*>& lists) {
        priority_queue<ListNode*, vector<ListNode*>, compare> pq;
        for (ListNode* l : lists) if (l) pq.push(l);
        ListNode head(0);
        ListNode *cur = &head;
        while (!pq.empty()) {
            cur->next = pq.top();
            pq.pop();
            cur = cur->next;
            if (cur->next) pq.push(cur->next);
        }
        return head.next;
    }
};

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004. Median of Two Sorted Arrays

Difficulty: Hard

Frequency: N/A

There are two sorted arrays nums1 and nums2 of size m and n respectively. Find the median of the two sorted arrays. The overall run time complexity should be O(log (m+n)).

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class Solution {
public:
    double findMedianSortedArrays(vector<int>& nums1, vector<int>& nums2) {
        int m = nums1.size(), n = nums2.size(), total = m + n;
        if (total % 2 == 1) return findK(nums1, 0, m, nums2, 0, n, total / 2 + 1);
        else return 0.5 * (findK(nums1, 0, m, nums2, 0, n, total / 2) +
            findK(nums1, 0, m, nums2, 0, n, total / 2 + 1));
        
    }
    int findK(vector<int>& nums1, int start1, int m, vector<int>& nums2, int start2, int n, int k) {
        if (m > n) return findK(nums2, start2, n, nums1, start1, m, k);
        if (m == 0) return nums2[start2 + k - 1];
        if (k == 1) return min(nums1[start1], nums2[start2]);
        int a = min(k / 2, m);
        int b = k - a;
        if (nums1[start1 + a - 1] <= nums2[start2 + b - 1]) return findK(nums1, start1 + a, m - a, nums2, start2, n, k - a);
        else return findK(nums1, start1, m, nums2, start2 + b, n - b, k - b);
    }
};

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215. Kth Largest Element in an Array

Difficulty: Medium

Frequency: N/A

Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note:
You may assume k is always valid, 1 ≤ k ≤ array’s length.


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class Solution {
public:
    int findKthLargest(vector<int>& nums, int k) {
        priority_queue<int> q;
        for (int n : nums) q.push(n);
        for (int i = 1; i < k; i++) q.pop();
        return q.top();
    }
};

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240. Search a 2D Matrix II

Difficulty: Medium

Frequency: N/A

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

  • Integers in each row are sorted in ascending from left to right.
  • Integers in each column are sorted in ascending from top to bottom.

For example,

Consider the following matrix:

[
  [1,   4,  7, 11, 15],
  [2,   5,  8, 12, 19],
  [3,   6,  9, 16, 22],
  [10, 13, 14, 17, 24],
  [18, 21, 23, 26, 30]
]

Given target = 5, return true.

Given target = 20, return false.


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class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        if (matrix.empty()) return false;
        int m = matrix.size(), n = matrix[0].size();
        int i = 0, j = n - 1;
        while (i < m && j >= 0) {
            if (matrix[i][j] == target) return true;
            else if (matrix[i][j] < target) i++;
            else j--;
        }
        return false;
    }
};

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241. Different Ways to Add Parentheses

Difficulty: Medium

Frequency: N/A

Given a string of numbers and operators, return all possible results from computing all the different possible ways to group numbers and operators. The valid operators are +,- and *.
Example 1Input: "2-1-1".

((2-1)-1) = 0
(2-(1-1)) = 2

Output: [0, 2]
Example 2Input: "2*3-4*5"

(2*(3-(4*5))) = -34
((2*3)-(4*5)) = -14
((2*(3-4))*5) = -10
(2*((3-4)*5)) = -10
(((2*3)-4)*5) = 10

Output: [-34, -14, -10, -10, 10]


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class Solution {
public:
    vector<int> diffWaysToCompute(string input) {
        vector<int> results;
        for (int i = 0; i < input.size(); i++) {
            char c = input[i];
            if (!isdigit(c)) {
                vector<int> left = diffWaysToCompute(input.substr(0, i));
                vector<int> right = diffWaysToCompute(input.substr(i + 1));
                for (int l : left) {
                    for (int r :right) {
                        switch(c) {
                            case '+': results.push_back(l + r); break;
                            case '-': results.push_back(l - r); break;
                            case '*': results.push_back(l * r); break;
                        }
                    }
                }
            }
        }
        if (results.empty()) results.push_back(stoi(input));
        return results;
    }
};

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