106. 从中序与后序遍历序列构造二叉树

dfs

class Solution {
    Map<Integer, Integer> map = new HashMap<>();
    public TreeNode buildTree(int[] inorder, int[] postorder) {
        for (int i = 0; i < inorder.length; i++) {
            map.put(inorder[i], i);
        }
        return build(inorder, postorder, 
                    0, inorder.length - 1,
                    0, postorder.length - 1);
    }

    public TreeNode build(int[] inorder, int[] postorder, 
                            int inLeft, int inRight,
                            int postLeft, int postRight) {
        if (inLeft > inRight) {
            return null;
        }
        TreeNode root = new TreeNode(postorder[postRight]);
        int index = map.get(postorder[postRight]);
        root.left = build(inorder, postorder,
                            inLeft, index - 1,
                            postLeft, postLeft + index - inLeft - 1);
        root.right = build(inorder, postorder,
                            index + 1, inRight,
                            postRight + index - inRight, postRight - 1);
        return root;
    }
}