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Solution007.java
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Solution007.java
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package com.portgas;
import java.util.HashMap;
import java.util.Map;
public class Solution007 {
public static class TreeNode {
public int val;
public TreeNode left;
public TreeNode right;
public TreeNode(int val) {
this.val = val;
}
}
/**
* 根据二叉树的前序遍历和中序遍历的结果,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。
**/
Map<Integer, Integer> map = new HashMap<>(); // 中序遍历,值-index的映射关系
public TreeNode reConstructBinaryTree(int[] pre, int[] in) {
if (pre == null || pre.length <= 0 || in == null || in.length <= 0) {
return null;
}
for (int i = 0; i < in.length; i++) {
map.put(in[i], i);
}
return reConstructBinaryTree(pre, 0, pre.length - 1, in, 0, in.length - 1);
}
public TreeNode reConstructBinaryTree(int[] pre, int preStart, int preEnd, int[] in, int inStart, int inEnd) {
if (preStart > preEnd) {
return null;
}
TreeNode root = new TreeNode(pre[preStart]);
int inIndex = map.get(pre[preStart]);
int leftSize = inIndex - inStart; // 左子树大小
root.left = reConstructBinaryTree(pre, preStart + 1, preStart + leftSize, in, inStart, inIndex - 1);
root.right = reConstructBinaryTree(in, preStart + leftSize + 1, preEnd, in, inIndex + 1, inEnd);
return root;
}
}