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从中序与后序遍历序列构造二叉树(Java)

大体思路：从后序数组中找到最后一个元素的值，即为当前节点并进行节点的创建，并在中序数组中找到该值所在索引（使用Map）。接着开始递归，后序数组倒着向前（所以递归应该先从右子树开始），当右子树遍历完（即不存在遍历区间时left>right）return。

问题分析：

1. 返回值：返回最终头节点；
2. 参数：由于需要遍历区间，所以定义两个参数left、right，至于map, postorder作为全局变量使用。

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {Map<Integer, Integer> map = new HashMap<>();int[] postorder;int postindex;public TreeNode buildTree(int[] inorder, int[] postorder) {this.postorder = postorder;postindex = postorder.length - 1;for(int i = 0; i < inorder.length; i++){map.put(inorder[i], i); // 通过值找索引（而且题目中表示值不会相同）}return treeBuild(0, postindex);}public TreeNode treeBuild(int left, int right //注意这个是inorder的左右区间 ){if(left > right){return null;}int rootval = postorder[postindex--];TreeNode root = new TreeNode(rootval); //创建节点int mid = map.get(rootval); //中序的切割点root.right = treeBuild(mid + 1, right);root.left = treeBuild(left, mid - 1);return root;}
}

从前序与中序遍历序列构造二叉树(Java)

思路分析：整体思路类似上一题，只不过这次先序遍历数组中的顺序是中左右，所以根节点是在最前边，同时递归时注意先左子树后右子树

/*** Definition for a binary tree node.* public class TreeNode {*     int val;*     TreeNode left;*     TreeNode right;*     TreeNode() {}*     TreeNode(int val) { this.val = val; }*     TreeNode(int val, TreeNode left, TreeNode right) {*         this.val = val;*         this.left = left;*         this.right = right;*     }* }*/
class Solution {Map<Integer, Integer> map = new HashMap<>();int[] preorder;int i = 0; // 记录先序数组中的中间节点的索引public TreeNode buildTree(int[] preorder, int[] inorder) {this.preorder = preorder;for(int i = 0; i < inorder.length; i++){map.put(inorder[i], i);}return fucTree(0, preorder.length - 1);}public TreeNode fucTree(int left, int right){if(left > right){return null;}int rootval = preorder[i++];TreeNode root = new TreeNode(rootval);int mid = map.get(rootval);root.left = fucTree(left, mid - 1);root.right = fucTree(mid + 1, right);return root;}
}