从中序与后序遍历序列构造二叉树

/**

• 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> hm = new HashMap<>();

public TreeNode buildTree(int[] inorder, int[] postorder) {
    for (int i = 0; i < inorder.length; ++i) {
        hm.put(inorder[i], i);
    }
    return dfs(inorder, 0, inorder.length - 1, postorder, 0, postorder.length - 1);
}

TreeNode dfs(int[] in, int il, int ir, int[] post, int pl, int pr) {
    if (il > ir) {
        return null;
    }
    int k = hm.get(post[pr]);
    TreeNode node = new TreeNode(post[pr]);
    node.left = dfs(in, il, k - 1, post, pl, pl + k - 1 - il);
    node.right = dfs(in, k + 1, ir, post, pl + k - il, pr - 1);
    return node;
}

}