105和106 写了1个多小时,终于悟了

513. 找树左下角的值

public int findBottomLeftValue(TreeNode root) {
        travel(root, 0);
        return answer;
    }

    private int maxDepth = -1;
    private int answer = 0;

    public void travel(TreeNode cur, int depth) {
        if (depth > maxDepth && cur.left == null && cur.right == null) {
            maxDepth = depth;
            answer = cur.val;
        }
        if (cur.left != null) {
            travel(cur.left, depth + 1);
        }
        if (cur.right != null) {
            travel(cur.right, depth + 1);
        }
    }

112. 路径总和

public boolean hasPathSum(TreeNode root, int targetSum) {
    if (root == null) {
            return false;
        }
        if (root.left == null && root.right == null) {
            targetSum -= root.val;
            return targetSum == 0;
        }
        boolean left = false;
        if (root.left != null) {
            left = hasPathSum(root.left, targetSum - root.val);
        }
        boolean right = false;
        if (root.right != null) {
            right = hasPathSum(root.right, targetSum - root.val);
        }
        return left || right;
    }

106. 从中序与后序遍历序列构造二叉树

        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 buildTree(inorder, 0, inorder.length, postorder, 0, inorder.length);
    }

    // 这个 inStart,代表的是,左边(右边)中序遍历的开始, poStart是 左边(右边) 后序遍历的开始
    public TreeNode buildTree(int[] inorder, int inStart, int inEnd, int[] postorder, int poStart, int poEnd) {
        if (inStart >= inEnd || poStart >= poEnd) {
            return null;
        }
        int mid = postorder[poEnd - 1];
        TreeNode root = new TreeNode(mid);
        Integer midIndex = map.get(mid);
        int leftInStart = inStart, leftInEnd = midIndex, leftLength = leftInEnd - leftInStart;
        int rightInStart = midIndex + 1, rightInEnd = inEnd;

        int leftPoStart = poStart, leftPoEnd = poStart + leftLength;
        int rightPoStart = leftPoEnd, rightPoEnd = poEnd - 1;
        root.left = buildTree(inorder, leftInStart, leftInEnd, postorder, leftPoStart, leftPoEnd);
        root.right = buildTree(inorder, rightInStart, rightInEnd, postorder, rightPoStart, rightPoEnd);
        return root;
    }