lk105.java

/**
 * 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 {
    //代替for循环寻找inorder中的index
    HashMap<Integer, Integer> valToIndex = new HashMap<>();

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

    TreeNode build(int[] preorder, int preStart, int preEnd,
                   int[] inorder, int inStart, int inEnd) {
        if (preStart > preEnd) {
            return null;
        }

        int rootVal = preorder[preStart];
        int index = valToIndex.get(rootVal);
        int leftSize = index - inStart;

        TreeNode root = new TreeNode(rootVal);
        root.left = build(preorder, preStart + 1, preStart + leftSize, inorder, inStart, index - 1);
        root.right = build(preorder, preStart + leftSize + 1, preEnd, inorder, index + 1, inEnd);
        return root;
    }
}