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package class032;
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import java.util.HashMap;
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// 测试链接：https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal
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public class Code02_ConstructBinaryTreeFromPreorderAndInorderTraversal {
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public static class TreeNode {
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int val;
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TreeNode left;
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TreeNode right;
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TreeNode(int val) {
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this.val = val;
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}
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}
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public static TreeNode buildTree1(int[] pre, int[] in) {
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if (pre == null || in == null || pre.length != in.length) {
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return null;
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}
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return process1(pre, 0, pre.length - 1, in, 0, in.length - 1);
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}
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// 只使用pre[L1...R1]这一段，作为这棵树先序遍历的结果
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// 只使用in [L2...R2]这一段，作为这棵树中序遍历的结果
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// 建立好这棵树，然后把头节点返回
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public static TreeNode process1(int[] pre, int L1, int R1, int[] in, int L2, int R2) {
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if (L1 > R1) {
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return null;
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}
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if (L1 == R1) {
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return new TreeNode(pre[L1]);
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}
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TreeNode head = new TreeNode(pre[L1]);
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int find = L2;
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while (in[find] != pre[L1]) {
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find++;
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}
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head.left = process1(pre, L1 + 1, L1 + find - L2, in, L2, find - 1);
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head.right = process1(pre, L1 + find - L2 + 1, R1, in, find + 1, R2);
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return head;
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}
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public static TreeNode buildTree2(int[] pre, int[] in) {
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if (pre == null || in == null || pre.length != in.length) {
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return null;
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}
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HashMap<Integer, Integer> valueIndexMap = new HashMap<>();
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for (int i = 0; i < in.length; i++) {
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valueIndexMap.put(in[i], i);
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}
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return process2(pre, 0, pre.length - 1, in, 0, in.length - 1, valueIndexMap);
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}
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// 只使用pre[L1...R1]这一段，作为这棵树先序遍历的结果
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// 只使用in [L2...R2]这一段，作为这棵树中序遍历的结果
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// 建立好这棵树，然后把头节点返回
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public static TreeNode process2(int[] pre, int L1, int R1, int[] in, int L2, int R2,
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HashMap<Integer, Integer> valueIndexMap) {
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if (L1 > R1) {
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return null;
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}
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if (L1 == R1) {
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return new TreeNode(pre[L1]);
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}
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TreeNode head = new TreeNode(pre[L1]);
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int find = valueIndexMap.get(pre[L1]);
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head.left = process2(pre, L1 + 1, L1 + find - L2, in, L2, find - 1, valueIndexMap);
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head.right = process2(pre, L1 + find - L2 + 1, R1, in, find + 1, R2, valueIndexMap);
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return head;
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}
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}
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