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二分法溢出的情况：

public int guessNumber(int n) {
        int leftP = 1;
        int rightP = n;
        int mid = 0;
        while(leftP < rightP){
		//皆是因为mid = (rightP + leftP) /2;
            mid = leftP + (rightP - leftP) / 2;
            if(guess(mid) == 0){
                return mid;
            }else if(guess(mid) == -1){
                rightP = mid;
            }else if(guess(mid) == 1){
                leftP = mid + 1;
            }
        }
        return rightP;
    }