Day 25 | 216. Combination Sum III | 17. Letter Combinations of a Phone Number

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

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LeetCode 216. Combination Sum III

Question Link

class Solution {
    List<List<Integer>> result = new ArrayList<>();
    List<Integer> path = new ArrayList<>();
    int sum = 0;
    
    public List<List<Integer>> combinationSum3(int k, int n) {
        backTracking(1, k, n);
        return result;
    }

    void backTracking(int startIndex, int k, int n){
        // pruning
        if(sum > n)
            return;
        if(path.size() == k && sum == n){
            result.add(new ArrayList<>(path));
            return;
        }    
        for(int i = startIndex; i <= 9 - (k - path.size()) + 1; i++){
            path.add(i);
            sum+=i;
            backTracking(i+1, k, n);
            path.remove(path.size() - 1);
            sum-=i;
        }
    }
}

• The number of nodes found: path.size()
• The number of nodes to find: k - path.size()
• The minimum length of [x, 9] should be k - path.size() for continued searching. We can get equation 9 - x + 1 = k - path.size()
  Solve the equation to get x = n - (k - path.size()) + 1

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LeetCode 17. Letter Combinations of a Phone Number

Question Link

class Solution {
    String[] numString = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    List<String> result = new ArrayList<>();
    StringBuilder path = new StringBuilder();

    public List<String> letterCombinations(String digits) {
        if(digits == null || digits.length() == 0)
            return result;
        backTracking(0, digits);
        return result;
    }

    void backTracking(int index, String digits){
        if(index == digits.length()){
            result.add(path.toString());
            return;
        }
        String str = numString[digits.charAt(index) - '0'];
        for(int i = 0; i < str.length(); i++){
            path.append(str.charAt(i));
            backTracking(index + 1, digits);
            path.deleteCharAt(path.length()-1);
        }
    }
}