LeetCode105.java

package problems;

import java.util.Deque;
import java.util.HashMap;
import java.util.LinkedList;
import java.util.Map;

/**
 * 递归
 */
public class LeetCode105 {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        int preLen = preorder.length;
        int inLen = inorder.length;
        if(preLen != inLen) {
            throw new RuntimeException("Incorrect input data");
        }
        Map<Integer, Integer> map = new HashMap<>(preLen);
        // 用空间换时间，快速知道元素对应的下标位置
        for (int i = 0; i < inLen; i++) {
            map.put(inorder[i], i);
        }
        return buildTree(preorder, 0, preLen - 1, map, 0, inLen - 1);
    }

    /**
     * @param preorder 前序遍历序列
     * @param preLeft  前序遍历序列子区间的左边界，可以取到
     * @param preRight 前序遍历序列子区间的右边界，可以取到
     * @param map      在中序遍历序列里，数值与下标的对应关系
     * @param inLeft   中序遍历序列子区间的左边界，可以取到
     * @param inRight  中序遍历序列子区间的右边界，可以取到
     * @return
     */
    private TreeNode buildTree(int[] preorder, int preLeft, int preRight, Map<Integer, Integer> map, int inLeft, int inRight) {
        if(preLeft > preRight || inLeft > inRight) {
            return null;
        }
        int rootValue = preorder[preLeft];
        TreeNode root = new TreeNode(rootValue);
        int pIndex = map.get(rootValue);

        root.left = buildTree(preorder, preLeft + 1, pIndex - inLeft + preLeft, map, inLeft, pIndex - 1);
        root.right = buildTree(preorder, pIndex - inLeft + preLeft + 1, preRight, map, pIndex + 1, inRight);
        return root;
    }
}

/**
 * 迭代
 */
class LeetCode105_1 {
    public TreeNode buildTree(int[] preorder, int[] inorder) {
        if (preorder == null || preorder.length == 0) {
            return null;
        }
        TreeNode root = new TreeNode(preorder[0]);
        Deque<TreeNode> stack = new LinkedList<TreeNode>();
        stack.push(root);
        int inorderIndex = 0;
        for (int i = 1; i < preorder.length; i++) {
            int preorderVal = preorder[i];
            TreeNode node = stack.peek();
            if (node.val != inorder[inorderIndex]) {
                node.left = new TreeNode(preorderVal);
                stack.push(node.left);
            } else {
                while (!stack.isEmpty() && stack.peek().val == inorder[inorderIndex]) {
                    node = stack.pop();
                    inorderIndex++;
                }
                node.right = new TreeNode(preorderVal);
                stack.push(node.right);
            }
        }
        return root;
    }
}

class LeetCode105_2 {
    private Map<Integer, Integer> indexMap;

    public TreeNode buildTree(int[] preorder, int[] inorder) {
        int n = preorder.length;
        // 构造哈希映射，帮助我们快速定位根节点
        indexMap = new HashMap<>();
        for (int i = 0; i < n; i++) {
            indexMap.put(inorder[i], i);
        }
        return MyBuildTree(preorder, inorder, 0, n - 1, 0, n - 1);
    }

    private TreeNode MyBuildTree(int[] preorder, int[] inorder, int preorder_left, int preorder_right, int inorder_left, int inorder_right) {
        if(preorder_left > preorder_right) {
            return null;
        }
        // 前序遍历第一个节点就是根节点
        int preorder_root = preorder_left;
        int inorder_root = indexMap.get(preorder[preorder_root]);
        TreeNode root = new TreeNode(preorder[preorder_root]);
        // 得到左子树的节点数目
        int size_left_subtree = inorder_root - inorder_left;
        root.left = MyBuildTree(preorder, inorder, preorder_left + 1, preorder_left + size_left_subtree, inorder_left, inorder_root - 1);
        root.right = MyBuildTree(preorder, inorder, preorder_left + size_left_subtree + 1, preorder_right, inorder_root + 1, inorder_right);
        return root;
    }
}