最近公共祖先（LCA树上倍增）

 

class TreeAncestor {

    int[][] dp; //预处理数组 dp[i][j] 表示i的第2^j个祖先
    // 倍增思想
    public TreeAncestor(int n, int[] parent) {
        dp = new int[n][(int)(Math.log(n) / Math.log(2)) + 1];
        for(int i = 0; i < n; i++) {
            dp[i][0] = parent[i];
        }
        for(int i = 0; i < n; i++) { // 先枚举状态j i都可 
            for(int j = 1; 1 << j < n; j++) {
                if(dp[i][j-1] != -1) {
                    dp[i][j] = dp[dp[i][j-1]][j-1];  
                } else {
                    dp[i][j] = -1;
                }
            }
        }
    }
    
    public int getKthAncestor(int node, int k) {
        if(node == -1 || k == 0) return node;
        int num = (int)(Math.log(k) / Math.log(2));
        return getKthAncestor(dp[node][num],k - (1 << num));
    }
}

/**
 * Your TreeAncestor object will be instantiated and called as such:
 * TreeAncestor obj = new TreeAncestor(n, parent);
 * int param_1 = obj.getKthAncestor(node,k);
 */