654.最大二叉树

public TreeNode constructMaximumBinaryTree(int[] nums) {return getMaxTree(nums,0,nums.length-1);
}public TreeNode getMaxTree(int[] nums, int head, int tail) {if (head > tail) {return null;}int maxIndex = getMaxIndex(nums, head, tail);int rootValue = nums[maxIndex];TreeNode root = new TreeNode(rootValue);root.left = getMaxTree(nums, head, maxIndex - 1);root.right = getMaxTree(nums, maxIndex + 1, tail);return root;}public int getMaxIndex(int[] nums, int head, int tail) {int maxIndex = head;for (int i = head; i <=tail; i++) {if (nums[i] > nums[maxIndex]) {maxIndex = i;}}return maxIndex;
}

617.合并二叉树

public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {if (root1 == null && root2 != null) {return root2;}if (root2 == null && root1 != null) {return root1;}if (root2 == null && root1 == null) {return null;}TreeNode treeNode = new TreeNode();treeNode.val = root1.val + root2.val;TreeNode dfsLeft = mergeTrees(root1.left, root2.left);TreeNode dfsRight = mergeTrees(root1.right, root2.right);treeNode.left = dfsLeft;treeNode.right = dfsRight;return treeNode;
}

700.二叉搜索树中的搜索 

public TreeNode searchBST(TreeNode root, int val) {int[] value = {val};List<TreeNode> list = new ArrayList<>();List<Integer> ans = new ArrayList<>();search(root, value, list, ans);if (!ans.contains(val)) {return null;}if (list.isEmpty()) {return null;}return list.get(0);
}public TreeNode search(TreeNode root, int[] val, List<TreeNode> list,List<Integer> ans) {if (root == null) {return null;}if (root.val == val[0]) {list.add(root);}ans.add(root.val);TreeNode leftTreeNode = search(root.left, val, list, ans);TreeNode rightTreeNode = search(root.right, val, list, ans);root.left = leftTreeNode;root.right = rightTreeNode;return root;
}