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ReConstructBinaryTree.java
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ReConstructBinaryTree.java
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package cn.sfturing.offer;
import cn.sfturing.entity.TreeNode;
/**
* 4.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。
*
* @author sfturing
* @date 2017年7月21日
*/
public class ReConstructBinaryTree {
public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
// 遍历得到结果
return reConstructBinaryTree(pre, 0, pre.length - 1, in, 0, in.length - 1);
}
// 前序遍历{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}
public TreeNode reConstructBinaryTree(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]);
// i - startIn等于左子树长度,加上括号便于理解
for (int i = startIn; i <= endIn; i++) {
if (pre[startPre] == in[i]) {
root.left = reConstructBinaryTree(pre, startPre + 1, startPre + (i - startIn), in, startIn, i - 1);
root.right = reConstructBinaryTree(pre, startPre + (i - startIn) + 1, endPre, in, i + 1, endIn);
}
}
return root;
}
public static void main(String[] arg) {
ReConstructBinaryTree binaryTree = new ReConstructBinaryTree();
int[] pre = {1, 2, 4, 7, 3, 5, 6, 8};
int[] in = {4, 7, 2, 1, 5, 3, 8, 6};
binaryTree.reConstructBinaryTree(pre, in);
}
}