Day 34 | 1005. Maximize Sum Of Array After K Negations | 134. Gas Station | 135. Candy

Day 1 | 704. Binary Search | 27. Remove Element | 35. Search Insert Position | 34. First and Last Position of Element in Sorted Array
Day 2 | 977. Squares of a Sorted Array | 209. Minimum Size Subarray Sum | 59. Spiral Matrix II
Day 3 | 203. Remove Linked List Elements | 707. Design Linked List | 206. Reverse Linked List
Day 4 | 24. Swap Nodes in Pairs| 19. Remove Nth Node From End of List| 160.Intersection of Two Lists
Day 6 | 242. Valid Anagram | 349. Intersection of Two Arrays | 202. Happy Numbe | 1. Two Sum
Day 7 | 454. 4Sum II | 383. Ransom Note | 15. 3Sum | 18. 4Sum
Day 8 | 344. Reverse String | 541. Reverse String II | 替换空格 | 151.Reverse Words in a String | 左旋转字符串
Day 9 | 28. Find the Index of the First Occurrence in a String | 459. Repeated Substring Pattern
Day 10 | 232. Implement Queue using Stacks | 225. Implement Stack using Queue
Day 11 | 20. Valid Parentheses | 1047. Remove All Adjacent Duplicates In String | 150. Evaluate RPN
Day 13 | 239. Sliding Window Maximum | 347. Top K Frequent Elements
Day 14 | 144.Binary Tree Preorder Traversal | 94.Binary Tree Inorder Traversal| 145.Binary Tree Postorder Traversal
Day 15 | 102. Binary Tree Level Order Traversal | 226. Invert Binary Tree | 101. Symmetric Tree
Day 16 | 104.MaximumDepth of BinaryTree| 111.MinimumDepth of BinaryTree| 222.CountComplete TreeNodes
Day 17 | 110. Balanced Binary Tree | 257. Binary Tree Paths | 404. Sum of Left Leaves
Day 18 | 513. Find Bottom Left Tree Value | 112. Path Sum | 105&106. Construct Binary Tree
Day 20 | 654. Maximum Binary Tree | 617. Merge Two Binary Trees | 700.Search in a Binary Search Tree
Day 21 | 530. Minimum Absolute Difference in BST | 501. Find Mode in Binary Search Tree | 236. Lowes
Day 22 | 235. Lowest Common Ancestor of a BST | 701. Insert into a BST | 450. Delete Node in a BST
Day 23 | 669. Trim a BST | 108. Convert Sorted Array to BST | 538. Convert BST to Greater Tree
Day 24 | 77. Combinations
Day 25 | 216. Combination Sum III | 17. Letter Combinations of a Phone Number
Day 27 | 39. Combination Sum | 40. Combination Sum II | 131. Palindrome Partitioning
Day 28 | 93. Restore IP Addresses | 78. Subsets | 90. Subsets II
Day 29 | 491. Non-decreasing Subsequences | 46. Permutations | 47. Permutations II
Day 30 | 332. Reconstruct Itinerary | 51. N-Queens | 37. Sudoku Solver
Day 31 | 455. Assign Cookies | 376. Wiggle Subsequence | 53. Maximum Subarray
Day 32 | 122. Best Time to Buy and Sell Stock II | 55. Jump Game | 45. Jump Game II

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LeetCode 1005. Maximize Sum Of Array After K Negations

Question Link

class Solution {
    public int largestSumAfterKNegations(int[] nums, int k) {
        // 1、Order the array by the absolute value from big to small.
        nums = IntStream.of(nums)
                .boxed()    // Covert the original stream to the boxed Stream
                .sorted((o1, o2) -> Math.abs(o2) - Math.abs(o1))
                .mapToInt(Integer::intValue).toArray();

        int len = nums.length;
        // 2、Traverse from front to back. Do negation and k-- when meeting negative numbers
        for(int i = 0; i < len; i++){
            if(nums[i] < 0 && k > 0){
                nums[i] = -nums[i];
                k--;
            }
        }
        // 3、If k > 0, repeatedly negate the smallest element
        if(k % 2 == 1)
            nums[len - 1] = - nums[len - 1];

        // 4、Sum up
        return Arrays.stream(nums).sum();
    }
}

• Order the array by the absolute value from big to small.
• Traverse from front to back. Do negation and k-- when meeting negative numbers
• If k > 0, repeatedly negate the smallest element
• Sum up

LeetCode 134. Gas Station

Question Link

class Solution {
    public int canCompleteCircuit(int[] gas, int[] cost) {
        int curSum = 0;
        int totalSum = 0;
        int index = 0;
        for(int i = 0; i < gas.length; i++){
            curSum += gas[i] - cost[i];
            totalSum += gas[i] - cost[i];
            if(curSum < 0){
                index = i + 1 % gas.length;
                curSum = 0;
            }
        }
        if(totalSum < 0)
            return -1;

        return index;
    }
}

• If the total gas minus the total consumption, then the car must be able to complete a lap. It demonstrates that the sum of rest gas(gas[i] - cost[i]) in every station must be bigger or equal to 0

LeetCode 135. Candy

Question Link

class Solution {
    public int candy(int[] ratings) {
        int len = ratings.length;
        int[] candyVec = new int[len];
        candyVec[0] = 1;

        // From left to right
        for(int i = 1; i < len; i++)
            candyVec[i] = (ratings[i] > ratings[i-1]) ? candyVec[i-1] + 1 : 1;
        
        // From right to left
        for(int i = len - 2; i >= 0; i--){
            if(ratings[i] > ratings[i+1])
                candyVec[i] = Math.max(candyVec[i], candyVec[i+1] + 1);
        }
        
        return Arrays.stream(candyVec).sum();
    }
}

We used two greedy strategies for this question:

• Traverse from left to right, only handle the situations the right rating value is larger than the left rating value
• Traverse from right to left, only handle the situations the left rating value is larger than the right rating value

In this way, we derive the global optimum from the local optimum