leetcode-1483-树节点的第K个祖先

 

 

 方法：倍增法dp  ACM经典

class TreeAncestor:

    def __init__(self, n: int, parent: List[int]):
        self.cols = 20      # log(50000) < 20

        self.dp = [[-1] * self.cols for _ in range(n)]
        for i in range(n):
            self.dp[i][0] = parent[i]
        # 动态规划设置祖先, dp[node][j] 表示 node 往前推第 2^j 个祖先
        for j in range(1, self.cols):
            for i in range(n):
                if self.dp[i][j-1] != -1:
                    self.dp[i][j] = self.dp[self.dp[i][j-1]][j-1]
        return

    def getKthAncestor(self, node: int, k: int) -> int:
        for i in range(self.cols - 1, -1, -1):
            if k & (1 << i):
                node = self.dp[node][i]
                if node == -1:
                    break
        return node