LeetCode --- 最大二叉樹
阿新 • • 發佈:2019-02-14
給定一個不含重複元素的整數陣列。一個以此陣列構建的最大二叉樹定義如下:
二叉樹的根是陣列中的最大元素。
左子樹是通過陣列中最大值左邊部分構造出的最大二叉樹。
右子樹是通過陣列中最大值右邊部分構造出的最大二叉樹。
通過給定的陣列構建最大二叉樹,並且輸出這個樹的根節點。
Example 1:
輸入: [3,2,1,6,0,5]
輸出: 返回下面這棵樹的根節點:
6
/ \
3 5
\ /
2 0
\
1
注意:
給定的陣列的大小在 [1, 1000] 之間。
public class constructMaximumBinaryTree {
@Test
public void constructMaximumBinaryTreeTest() {
int[] nums = new int[]{3,2,1,6,0,5};
TreeNode treeNode = constructMaximumBinaryTree(nums);
Assert.assertEquals(6 , treeNode.val);
Assert.assertEquals(3, treeNode.left.val);
Assert.assertEquals(5, treeNode.right.val);
Assert.assertEquals(null , treeNode.left.left);
Assert.assertEquals(2, treeNode.left.right.val);
Assert.assertEquals(null, treeNode.left.right.left);
Assert.assertEquals(1, treeNode.left.right.right.val);
Assert.assertEquals(0, treeNode.right.left.val);
Assert.assertEquals(null, treeNode.right.right);
}
public class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) { val = x; }
}
public TreeNode constructMaximumBinaryTree(int[] nums) {
if (nums == null || nums.length == 0) return null;
int maxInt = Arrays.stream(nums).max().getAsInt();
TreeNode root = new TreeNode(maxInt);
int[] left = getLeftArray(nums, getIndexInArray(nums, maxInt));
int[] rigt = getRightArray(nums, getIndexInArray(nums, maxInt));
root.left = constructMaximumBinaryTree(left);
root.right = constructMaximumBinaryTree(rigt);
return root;
}
@Test
public void getLeftArrayTest() {
int[] nums = new int[]{3,2,1,6,0,5};
Assert.assertArrayEquals(new int[]{3,2,1}, getLeftArray(nums, 3));
}
private int[] getLeftArray(int[] nums, int index) {
if (index == -1) {
return null;
}
int[] left = new int[index];
Arrays.setAll(left, i -> nums[i]);
return left;
}
@Test
public void getRightArrayTest() {
int[] nums = new int[]{3,2,1,6,0,5};
Assert.assertArrayEquals(new int[]{0,5}, getRightArray(nums, 3));
}
private int[] getRightArray(int[] nums, int index) {
if (index == -1) {
return null;
}
int[] right = new int[nums.length - index - 1];
Arrays.setAll(right, i -> nums[index + i + 1]);
return right;
}
@Test
public void getIndexInArrayTest() {
int[] nums = new int[]{3,2,1,6,0,5};
Assert.assertEquals(3, getIndexInArray(nums, 6));
}
private int getIndexInArray(int[] nums, int maxInt) {
for (int i = 0; i < nums.length; i++) {
if (maxInt == nums[i]) {
return i;
}
}
return -1;
}
}