98.验证二叉搜索树

class Solution {public boolean isValidBST(TreeNode root) {Stack<TreeNode> stack = new Stack<>();TreeNode pre = null;if(root != null){stack.add(root);}while(!stack.isEmpty()){TreeNode curr = stack.peek();if(curr != null){stack.pop();if(curr.right != null){stack.add(curr.right);}stack.add(curr);stack.add(null);if(curr.left != null){stack.add(curr.left);}}else {stack.pop();TreeNode temp = stack.pop();if(pre != null && pre.val >= temp.val){return false;}pre = temp;}}return true;}
}

700. 二叉搜索树中的搜索

class Solution {public TreeNode searchBST(TreeNode root, int val) {if(root == null || root.val == val){return root;}TreeNode left = searchBST(root.left,val);if(left != null){return left;}return searchBST(root.right,val);}
}

617. 合并二叉树

class Solution {public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {if(root1 == null){return root2;}if(root2 == null){return root1;}root1.val += root2.val;root1.left = mergeTrees(root1.left,root2.left);root1.right = mergeTrees(root1.right,root2.right);return root1;}
}

654. 最大二叉树

class Solution {public TreeNode constructMaximumBinaryTree(int[] nums) {
if(nums.length == 0){return null;}int max = 0;Map<Integer,Integer> map = new HashMap<>();for(int i = 0 ; i < nums.length ; i++){max = Math.max(max,nums[i]);map.put(nums[i],i);}int index = map.get(max);TreeNode root = new TreeNode(max);int[] leftNums = new int[index];int[] rightNums = new int[nums.length-1-index];for(int i = 0 ; i < index; i ++){leftNums[i] = nums[i];}for(int i = index+1 ; i < nums.length ; i++){rightNums[i-(index+1)] = nums[i];}root.left = constructMaximumBinaryTree(leftNums);root.right = constructMaximumBinaryTree(rightNums);return root;}
}