题目描述
输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。
package com.happy.algorithm.codinginterviews;
/**
*/
public class ReconstructTree {
public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
if (in == null || pre == null || pre.length == 0 || pre.length != in.length) {
return null;
}
int prePos = 0;
int start = 0;
int end = pre.length - 1;
TreeNode root = construct(pre, in, prePos, start, end);
return root;
}
private TreeNode construct(int[] pre, int[] in, int prePos, int start, int end) {
TreeNode root = null;
if (start <= end) {
root = new TreeNode(pre[prePos]);
int pos = findPos(in, pre[prePos], start, end);
root.left = construct(pre, in, prePos + 1, start, pos - 1);
root.right = construct(pre, in, prePos + pos - start + 1, pos + 1, end);
}
return root;
}
private int findPos(int[] in, int rootValue, int start, int end) {
for (int i = start; i <= end; i++) {
if (in[i] == rootValue) {
return i;
}
}
return 0;
}
}
class PreSolution {
public TreeNode buildTree(int[] preorder, int[] inorder) {
if (preorder == null || inorder == null) {
return null;
} else {
TreeNode root = reConstructBinaryTree2(preorder, 0, preorder.length - 1, inorder, 0, inorder.length - 1);
return root;
}
}
private TreeNode reConstructBinaryTree2(int[] pre, int startPre, int endPre, int[] in, int startIn, int endIn) {
if (startPre > endPre || startIn > endIn) {
return null;
}
TreeNode root = new TreeNode(pre[startPre]);
for (int i = startIn; i <= endIn; i++) {
if (in[i] == pre[startPre]) {
if (i == startIn) {
root.left = null;
} else {
root.left = reConstructBinaryTree2(pre, startPre + 1, startPre + i - startIn, in, startIn, i - 1);
}
if (i == endIn) {
root.right = null;
} else {
root.right = reConstructBinaryTree2(pre, startPre + i - startIn + 1, endPre, in, i + 1, endIn);
}
break;
}
}
return root;
}
}
class TreeNode {
int val;
TreeNode left;
TreeNode right;
TreeNode(int x) {
val = x;
}
}
