树的算法。开篇，主要是递归

Author: h2pl

src/数据结构/树/TreeNode.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/19.
+ */
+public class TreeNode {
+    int val;
+    TreeNode left;
+    TreeNode right;
+    TreeNode(int x) { val = x; }
+}

src/数据结构/树/两节点的最长路径.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 两节点的最长路径 {
+}

src/数据结构/树/二叉查找树的最近公共祖先.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 二叉查找树的最近公共祖先 {
+}

src/数据结构/树/二叉树的最近公共祖先.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 二叉树的最近公共祖先 {
+}

src/数据结构/树/从有序数组中构造二叉查找树.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 从有序数组中构造二叉查找树 {
+}

src/数据结构/树/修剪二叉查找树.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 修剪二叉查找树 {
+}

src/数据结构/树/判断路径和是否等于一个数.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 判断路径和是否等于一个数 {
+    //本题有个坑。空树一定返回false。所以要判断左右子树为空并且val = sum。
+    //而不能直接判断sum = 0 && root = null
+    public boolean hasPathSum(TreeNode root, int sum) {
+        if (root == null) return false;
+        if (root.left == null && root.right == null && root.val == sum)return true;
+        sum -= root.val;
+        boolean left = hasPathSum(root.left, sum);
+        boolean right = hasPathSum(root.right, sum);
+        return left || right;
+    }
+}

src/数据结构/树/子树.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 子树 {
+}

src/数据结构/树/平衡树.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 平衡树 {
+    public boolean isBalanced(TreeNode root) {
+        if (root == null)return true;
+        int left = maxDepth(root.left);
+        int right = maxDepth(root.right);
+        if (Math.abs(left - right) > 1)return false;
+        return isBalanced(root.left) && isBalanced(root.right);
+    }
+    public int maxDepth(TreeNode root) {
+        if (root == null)return 0;
+        int left = maxDepth(root.left);
+        int right = maxDepth(root.right);
+        return (left > right ? left : right) + 1;
+    }
+}

src/数据结构/树/归并两棵树.java

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+package 数据结构.树;
+
+/**
+ * Created by 周杰伦 on 2018/4/20.
+ */
+public class 归并两棵树 {
+    public TreeNode mergeTrees(TreeNode t1, TreeNode t2) {
+        if (t1 == null)return t2;
+        if (t2 == null)return t1;
+        TreeNode t = new TreeNode(t1.val);
+        t.val = t1.val + t2.val;
+        t.left = mergeTrees(t1.left, t2.left);
+        t.right = mergeTrees(t1.right, t2.right);
+        return t;
+    }
+
+}