Data Structure and Algorithm

• Recursion

  layer by layer

  private xxxx recursion(level, param1, param2, ...) {
      // recursion terminator
      if (level > MAX_LEVEL) {
          process_result;
          return xxx;
      }
      
      // process logic in current level
      process(level, data...);
      
      // drill down
      recursion(level + 1, p1, p2, ...);
      
      // reverse the current level status if needed
  }
  

• 70. Climbing Stairs

  You are climbing a staircase. It takes n steps to reach the top.

  Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?

  Example 1:

  Input: n = 2
  Output: 2
  Explanation: There are two ways to climb to the top.
  1. 1 step + 1 step
  2. 2 steps
  

  Example 2:

  Input: n = 3
  Output: 3
  Explanation: There are three ways to climb to the top.
  1. 1 step + 1 step + 1 step
  2. 1 step + 2 steps
  3. 2 steps + 1 step
  

  Constraints:

  • 1 <= n <= 45

  class Solution {
      public int climbStairs(int n) {
          Map<Integer, Integer> map = new HashMap<>();
          map.put(1, 1);
          map.put(2, 2);
          return helper(map, n);
      }
  
      private int helper(Map<Integer, Integer> map, int n) {
          if (map.containsKey(n)) {
              return map.get(n);
          }
          int nStair = helper(map, n-2) + helper(map, n-1);
          map.put(n, nStair);
          return nStair;
      }
  }
  

• 22. Generate Parentheses

  Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

  Example 1:

  Input: n = 3
  Output: ["((()))","(()())","(())()","()(())","()()()"]
  

  Example 2:

  Input: n = 1
  Output: ["()"]
  

  Constraints:

  • 1 <= n <= 8

  class Solution {
      public List<String> generateParenthesis(int n) {
          List<String> res = new ArrayList<>();
          helper(res, 0, 0, n*2, "");
          return res;
      }
  
      private void helper(List<String> list, int l, int r, int max, String s) {
          if (l + r == max) {
              list.add(s);
              return ;
          }
          if (l < max / 2) helper(list, l + 1, r, max, s + "(");
          if (l > r) helper(list, l, r + 1, max, s + ")");
      }
  }
  

• 98. Validate Binary Search Tree

  Given the root of a binary tree, determine if it is a valid binary search tree (BST).

  A valid BST is defined as follows:

  • The left subtree of a node contains only nodes with keys less than the node's key.
  • The right subtree of a node contains only nodes with keys greater than the node's key.
  • Both the left and right subtrees must also be binary search trees.

  Example 1:

  

  Input: root = [2,1,3]
  Output: true
  

  Example 2:

  

  Input: root = [5,1,4,null,null,3,6]
  Output: false
  Explanation: The root node's value is 5 but its right child's value is 4.
  

  Constraints:

  • The number of nodes in the tree is in the range [1, 104].
  • -231 <= Node.val <= 231 - 1

  // inorder
  class Solution {
      private long cur = Long.MIN_VALUE;
      boolean flag = true;
      public boolean isValidBST(TreeNode root) {
          if (root == null) return true;
          if (flag && root.left != null) isValidBST(root.left);
          if (root.val <= cur) flag = false;
          cur = root.val;
          if (flag && root.right != null) isValidBST(root.right);
          return flag;
      }
  }
  

  // min bound max bound
  class Solution {
      public boolean isValidBST(TreeNode root) {
          if (root == null) return true;
          return recur(root, Long.MIN_VALUE, Long.MAX_VALUE);
      }
  
      private boolean recur(TreeNode root, long min, long max) {
          if (root.val <= min || root.val >= max) {
              return false;
          }
          boolean left = true, right = true;
          if (root.left != null) left = recur(root.left, min, root.val);
          if (root.right != null) right = recur(root.right, root.val, max);
          return left && right;
      }
  }
  

• 226. Invert Binary Tree

  Given the root of a binary tree, invert the tree, and return its root.

  Example 1:

  

  Input: root = [4,2,7,1,3,6,9]
  Output: [4,7,2,9,6,3,1]
  

  Example 2:

  

  Input: root = [2,1,3]
  Output: [2,3,1]
  

  Example 3:

  Input: root = []
  Output: []
  

  Constraints:

  • The number of nodes in the tree is in the range [0, 100].
  • -100 <= Node.val <= 100

  class Solution {
      public TreeNode invertTree(TreeNode root) {
          if (root == null) return null;
          TreeNode left = invertTree(root.left);
          TreeNode right = invertTree(root.right);
          root.left = right;
          root.right = left;
          return root;
      }
  }
  

• 104. Maximum Depth of Binary Tree

  Given the root of a binary tree, return its maximum depth.

  A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

  Example 1:

  

  Input: root = [3,9,20,null,null,15,7]
  Output: 3
  

  Example 2:

  Input: root = [1,null,2]
  Output: 2
  

  Example 3:

  Input: root = []
  Output: 0
  

  Example 4:

  Input: root = [0]
  Output: 1
  

  Constraints:

  • The number of nodes in the tree is in the range [0, 104].
  • -100 <= Node.val <= 100

  class Solution {
      public int maxDepth(TreeNode root) {
          if (root == null) return 0;
          return 1 + Math.max(maxDepth(root.left), maxDepth(root.right));
      }
  }
  

• 111. Minimum Depth of Binary Tree

  Given a binary tree, find its minimum depth.

  The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.

  Note: A leaf is a node with no children.

  Example 1:

  

  Input: root = [3,9,20,null,null,15,7]
  Output: 2
  

  Example 2:

  Input: root = [2,null,3,null,4,null,5,null,6]
  Output: 5
  

  Constraints:

  • The number of nodes in the tree is in the range [0, 105].
  • -1000 <= Node.val <= 1000

  class Solution {
      public int minDepth(TreeNode root) {
          if (root == null) return 0;
          if (root.left == null && root.right == null) {
              return 1;
          }
          int left = root.left == null ? Integer.MAX_VALUE : minDepth(root.left);
          int right = root.right == null ? Integer.MAX_VALUE : minDepth(root.right);
          return 1 + Math.min(left, right);
      }
  }
  

• 236. Lowest Common Ancestor of a Binary Tree

  Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree.

  According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).”

  Example 1:

  

  Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
  Output: 3
  Explanation: The LCA of nodes 5 and 1 is 3.
  

  Example 2:

  

  Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
  Output: 5
  Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant of itself according to the LCA definition.
  

  Example 3:

  Input: root = [1,2], p = 1, q = 2
  Output: 1
  

  Constraints:

  • The number of nodes in the tree is in the range [2, 105].
  • -109 <= Node.val <= 109
  • All Node.val are unique.
  • p != q
  • p and q will exist in the tree.

  class Solution {
      public TreeNode lowestCommonAncestor(TreeNode root, TreeNode p, TreeNode q) {
          if(root == null || root == p || root == q) return root;
          TreeNode left = lowestCommonAncestor(root.left, p, q);
          TreeNode right = lowestCommonAncestor(root.right, p, q);
          if(left == null) return right;
          if(right == null) return left;
          return root;
      }
  }
  

• 105. Construct Binary Tree from Preorder and Inorder Traversal

  Given two integer arrays preorder and inorder where preorder is the preorder traversal of a binary tree and inorder is the inorder traversal of the same tree, construct and return the binary tree.

  Example 1:

  

  Input: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
  Output: [3,9,20,null,null,15,7]
  

  Example 2:

  Input: preorder = [-1], inorder = [-1]
  Output: [-1]
  

  Constraints:

  • 1 <= preorder.length <= 3000
  • inorder.length == preorder.length
  • -3000 <= preorder[i], inorder[i] <= 3000
  • preorder and inorder consist of unique values.
  • Each value of inorder also appears in preorder.
  • preorder is guaranteed to be the preorder traversal of the tree.
  • inorder is guaranteed to be the inorder traversal of the tree.

  class Solution {
      public TreeNode buildTree(int[] preorder, int[] inorder) {
          int len = preorder.length;
          if (len == 0) return null;
          return recur(preorder, 0, len - 1, inorder, 0, len - 1);
      }
  
      private TreeNode recur(int[] preorder, int pl, int pr, int[] inorder, int il, int ir) {
          if (pl > pr) return null;
          TreeNode root = new TreeNode(preorder[pl]);
          int i = il;
          for ( ; i <= ir; i++) {
              if (preorder[pl] == inorder[i]) {
                  break;
              }
          }
          int leftNums = i - il;
          TreeNode left = recur(preorder, pl + 1, pl + 1 + leftNums - 1, inorder, il, i - 1);
          TreeNode right = recur(preorder, pl + 1 + leftNums, pr, inorder, i + 1, ir);
          root.left = left;
          root.right = right;
          return root;
      }
  }
  

• 77. Combinations

  Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

  You may return the answer in any order.

  Example 1:

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

  Example 2:

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

  Constraints:

  • 1 <= n <= 20
  • 1 <= k <= n

  class Solution {
      public List<List<Integer>> combine(int n, int k) {
          List<List<Integer>> res = new ArrayList<>();
          List<Integer> list = new ArrayList<>();
          recur(res, n, k, list, 1);
          return res;
      }
  
      private void recur(List<List<Integer>> lists, int n, int k, List<Integer> list, int num) {
          if (list.size() == k) {
              lists.add(new ArrayList(list));
              return ;
          }
          for (int i = num; i <= n; i++) {
              list.add(i);
              recur(lists, n, k, list, i + 1);
              list.remove(list.size() - 1);
          }
      }
  }