find peak element

Author: fishercoder1534

MEDIUM/src/medium/FindPeakElement.java

@@ -0,0 +1,55 @@
+package medium;
+
+public class FindPeakElement {
+
+    /**
+     * On discuss, this post has very good explanation about an O(logn) solution:
+     * https://discuss.leetcode.com/topic/29329/java-solution-and-explanation-using-invariants
+     * 
+     * Basically, we need to keep this invariant:
+     * nums[left] > nums[left-1], then we could return left as the result
+     * or nums[right] > nums[right+1], then we could return right as the result
+     */
+    public static int findPeakElement_Ologn(int[] nums) {
+        
+        if(nums == null || nums.length == 0) return 0;
+        int left = 0, right = nums.length-1;
+        while(left+1 < right){
+            int mid = left + (right-left)/2;
+            if(nums[mid] < nums[mid+1]){
+                left = mid;
+            } else {
+                right = mid;
+            }
+        }
+        return (left == nums.length-1 || nums[left] > nums[left+1]) ? left : right;
+    
+    }
+    
+    /**My original O(n) solution.*/
+    public static int findPeakElement(int[] nums) {
+        if(nums == null || nums.length == 0) return 0;
+        int n = nums.length, result = 0;
+        for(int i = 0; i < n; i++){
+            if(i == 0 && n > 1 && nums[i] > nums[i+1]){
+                result = i;
+                break;
+            } else if(i == n-1 && i > 0 && nums[i] > nums[i-1]){
+                result = i;
+                break;
+            } else if(i > 0 && i < n-1 && nums[i] > nums[i-1] && nums[i] > nums[i+1]){
+                result = i;
+                break;
+            }
+        }
+        return result;
+    }
+
+    public static void main(String...strings){
+//        int[] nums = new int[]{1,2};
+//        int[] nums = new int[]{1};
+        int[] nums = new int[]{1,2,3,1};
+//        System.out.println(findPeakElement(nums));
+        System.out.println(findPeakElement_Ologn(nums));
+    }
+}

README.md

@@ -25,6 +25,7 @@
 |200|[Number of Islands](https://leetcode.com/problems/number-of-islands/)|[Union Find](../../blob/master/MEDIUM/src/medium/NumberOfIslandsUnionFind.java) [DFS](../../blob/master/MEDIUM/src/medium/NumberofIslandsDFS.java)| O(m*n)|O(m*n) | Medium| Union Find, DFS
 |189|[Rotate Array](https://leetcode.com/problems/rotate-array/)|[Solution](../../blob/master/EASY/src/easy/RotateArray.java)| O(n)|O(n), could be optimized to O(1) | Easy
 |173|[Binary Search Tree Iterator](https://leetcode.com/problems/binary-search-tree-iterator/)|[Queue](../../blob/master/MEDIUM/src/medium/BSTIterator_using_q.java) [Stack](../../blob/master/MEDIUM/src/medium/BSTIterator_using_stack.java)| O(1) |O(h) | Medium|
+|162|[Find Peak Element](https://leetcode.com/problems/find-peak-element/)|[Solution](../../blob/master/MEDIUM/src/medium/FindPeakElement.java) | O(1) |O(logn)/O(n) | Binary Search|
 |140|[Word Break II](https://leetcode.com/problems/word-break-ii/)|[Solution](../../blob/master/HARD/src/hard/WordBreakII.java)| ? |O(n^2) | Hard| Backtracking/DFS
 |139|[Word Break](https://leetcode.com/problems/word-break/)|[Solution](../../blob/master/MEDIUM/src/medium/WordBreak.java)| O(n^2)|O(n) | Medium| DP
 |133|[Clone Graph](https://leetcode.com/problems/clone-graph/)|[Solution](../../blob/master/MEDIUM/src/medium/CloneGraph.java)| O(n)|O(n) | Medium| HashMap, BFS