public boolean verifySequenceOfBST(int [] sequence) {
        if(sequence == null || sequence.length < 2) return true;
        return verifySequenceOfBST(sequence, 0, sequence.length - 1);
    }
    private boolean verifySequenceOfBST(int[] sequence, int st, int ed) {
        if(ed - st <= 1) return true;
        int rootVal = sequence[ed];
        int curIndex = st;
        while(curIndex < ed && sequence[curIndex] <= rootVal) ++curIndex;  //左子树都小于等于rootVal
        for(int i = curIndex; i < ed; i++) {  //判断右子树大于等于rootVal
            if(sequence[i] < rootVal) {   //不符合条件返回false
                return false;
            }
        }
        //drill down
        return verifySequenceOfBST(sequence, st, curIndex-1) && verifySequenceOfBST(sequence, curIndex, ed - 1);
    }