/*
 * @lc app=leetcode id=162 lang=java
 *
 * [162] Find Peak Element
 */

// @lc code=start
class Solution {
    public int findPeakElement(int[] nums) {
        int l = 0, r = nums.length-1, m;
        while(l <= r) {
            m = l + (r-l)/2;
            if(isPeak(m, nums)) {
                return m;
            } else if(isIncreasing(m, nums)) {
                l = m + 1;
            } else {
                r = m - 1;
            }
        }
        throw new RuntimeException("input is not valid");
    }

    private boolean isPeak(int i, int[] nums) {
        boolean left = false, right = false;
        if(i == 0) {
            left = true;
        } else {
            left = nums[i] > nums[i-1];
        }
        if(i == nums.length-1) {
            right = true;
        } else {
            right = nums[i] > nums[i+1];
        }
        return left && right;
    }

    private boolean isIncreasing(int i, int[] nums) {
        if(isPeak(i, nums)) {
            throw new RuntimeException("unexpected input");
        }
        return i>0?nums[i]-nums[i-1]>0:nums[i+1]-nums[i]>0;
    }
}
// @lc code=end